WHO/MSA/PSA/97.4
English Only
Distr.: General

PROGRAMME ON SUBSTANCE ABUSE

Cannabis:a health perspective
and research agenda

DIVISION OF MENTAL HEALTH AND
PREVENTION OF SUBSTANCE ABUSEWORLD HEALTH ORGANIZATION

Abstract

The use of cannabis, a psychoactive substance under international control, is widespread throughout the world.
Reliable information on the actual and potential health consequences of cannabis use is thus an important input
into health policy analysis and for the development of national and international drug control strategies.
The last WHO report on this topic was issued jointly with the Addiction Research Foundation of Ontario in 1981
(ARF/WHO Scientific Meeting on Adverse Health and Behavioural Consequences of Cannabis Use). In theintervening years, there have been many requests for an updated WHO report on the health consequences ofcannabis use.
In response to these requests, WHO convened a group of scientific experts on cannabis in Geneva in November
1993. The present report is the end-product of a review and update process which started at that meeting. This
report provides a review and summary of current knowledge about cannabis use and health effects, and is likely
to be relevant for policy makers, public health officials, educators, and others concerned with health promotion.
Epidemiological studies from Australia, Canada, Europe, and the USA have indicated an increase in the prevalenceof cannabis use by young people over the last decade. In other regions, mostly in developing countries, data
available is more scarce, making it difficult to draw any conclusions about the general levels of cannabis use inthese countries.
There have been significant advances in research over the last 15 years. These include: basic research on themechanism of action of cannabinoids (of which -9-tetrahydrocannabinol or THC is the most potent), themolecular structure needed for such action, the discovery of a specific receptor molecule to which the cannabinoid
molecule attaches in brain cells and other tissue sites, the discovery of a natural chemical substance in the brainthat normally acts on those receptor sites, and the mapping of the receptor sites in various parts of the brain andelsewhere in the body. Cannabis acutely impairs cognitive development and psychomotor performance, which
increases the risk of motor vehicle accidents among those who drive intoxicated by cannabis. There has also been
substantial progress in understanding the chronic effects of cannabis on the respiratory system and on various
types of cells in the body’s immune system. Chronically, there are selective impairments of cognitive functioning,
and a dependence syndrome may develop. Chronic cannabis use may also exacerbate schizophrenia in affectedindividuals. On the other hand, several studies have demonstrated therapeutic effects of THC for nausea andvomiting in advanced stages of cancer and AIDS and studies on other therapeutic uses are underway.
There is a clear need for both epidemiological and applied research on cannabis and its derivatives. There areimportant gaps in knowledge about the health consequences of cannabis use which need to be addressed by wellcontrolled studies, including data on the patterns and consequences of cannabis use in developing countries, thechronic adverse effects of cannabis use and on the relative effectiveness of cannabinoids for medical use.

8 World Health Organization, 1997
This document is not a formal publication of the World Health Organization (WHO), and all rights are reserved by theOrganization. The document may, however, be freely reviewed, abstracted, reproduced and translated, in part or in whole, but
not for sale nor for use in conjunction with commercial purposes.
The views expressed in documents by named authors are solely the responsibility of those authors.

ii

Contents

Acknowledgements ............................................................................................................................................

v

1.

Why this report now?......................................................................................................... .................

1

2.

Cannabis and health: some issues about inference ..........................................................................

2

3.

Epidemiology of cannabis use.............................................................................................................

4

4.

Chemistry and pharmacology ............................................................................................................

11

5.

Effects on the brain and behaviour........................................................................................... .........

14

6.

Effect on the respiratory system ........................................................................................................

20

7.

Effects on endocrine and reproductive system.................................................................................

228.

Effects on intrauterine and postnatal development .........................................................................

24

9.

Effects on cell nuclei.............................................................................................................................

26

10.

Effects on immune system ...................................................................................................................

27

11.

Effects on other organ systems ...........................................................................................................

27

12.

Therapeutic uses...................................................................................................................................

28

13.

Comparing cannabis with other drugs ..............................................................................................

29

14.

Summary ...............................................................................................................................................

30

15.

Recommendations for future research ..............................................................................................

31

References...........................................................................................................................................................

34

ANNEXES:
Annex 1

Annex 2

List of participants of the WHO project meeting on health implicationsof cannabis use, Geneva, 22-24 May 1995...........................................................................

47

Background papers prepared for the WHO project meeting on health
implications of cannabis use, Geneva, 22-24 May 1995.....................................................

49

iii

Acknowledgements
Many individuals have assisted the Programme on Substance Abuse (PSA) in the preparation of this document. The
contribution of the members of the WHO Expert Working Group on the Health Effects of Cannabis Use (see Annex
1) and the authors of the background documents for the Working Group meeting in May 1995 (see Annex 2) isgratefully acknowledged.
Based on the deliberation of the Working Group meeting, an editorial group consisting of Drs Harold Kalant,
William Corrigall and Reginald Smart prepared a summary report which has served as the basis for this document.
Their contribution was substantial and much appreciated. Subsequent drafts were reviewed by over 100 external
reviewers, including selected scientists of the WHO Expert Advisory Panel on Drug Dependence and Alcohol
Problems, and all WHO Collaborating Centres in the field of substance abuse. We wish to specifically acknowledge
their input, as well as the contribution of Dr Wayne Hall and of the various experts at the US National Institute on
Drug Abuse (NIDA) who have commented on various drafts of this report. Several helpful comments andsuggestions were also made by the United Nations International Drug Control Programme (UNDCP).

iv

WHO/MSA/PSA/97.4
page 1

1.

Why this report now ?

1.1

Need for the report

The goal of the World Health Organization (WHO) is to promote the highest possible level of health for all peoplesof the world. Psychoactive substances are a major cause of disease and injury in all regions of the world and asignificant impediment to progress of >health for all= strategies.
The use of cannabis, a psychoactive substance under international control, is widespread throughout the world.
Reliable information of the actual and potential health consequences of cannabis use, including the costs and benefitsof different interventions, is thus an important input into health policy analysis and for the development of national
and international drug control strategies.
The last WHO report on this topic was issued jointly with the Addiction Research Foundation of Ontario in 1981
(ARF/WHO Scientific Meeting on Adverse Health and Behavioural Consequences of Cannabis Use). In theintervening years, there have been many requests for an updated WHO report on the health consequences of cannabisuse.
In response to these requests, WHO convened a group of scientific experts on cannabis in Geneva in November
1993. The participants at this meeting (listed in Annex 1) agreed that an updated report on the health consequencesof cannabis use should be prepared, and adopted a two-stage plan to produce such a report. First, scientists werecommissioned to produce extensive literature reviews in the form of background papers on various topics, and thesewere reviewed by other experts. The authors and titles of the background documents are listed in Annex 2. Second,
based on these reviews, a summary report was drafted by the original group of experts during a second meeting heldin Geneva from 22-24 May 1995.
The draft summary report was then circulated to selected scientists of the WHO Expert Advisory Panel for Drug
Dependence and Alcohol Problems, WHO Collaborating Centres, as well as other scientists and various technicalunits within WHO. Subsequently, the report was revised on the basis of these comments and in collaboration withthe authors of the relevant sections of the summary report. The present report is thus a collaborative review basedon inputs from scientists and public health specialists in all regions of the world.
1.2

Purpose and content of the report

This report provides a review and summary of current knowledge about cannabis use and health effects, and is likely
to be relevant for policy makers, public health officials, educators, and others concerned with health promotion. The
report is intended less for the use of research scientists and clinical experts, who usually have access to detailed
information and studies through specialty journals and monographs, and computerized bibliographic services. In
preparing this report, an effort has been made to summarize knowledge without using excessively technical language
where possible, or with only essential bibliographic documentation, and with an emphasis on the major changes inknowledge in the past fifteen years and their potential implications.
There have indeed been significant advances in some areas in the last fifteen years. These include: basic research on
the mechanism of action of cannabinoids, the molecular structure needed for such action, the discovery of a specificreceptor molecule to which the cannabinoid molecule attaches in brain cells and other tissue sites, the discovery ofa natural chemical substance in the brain that normally acts on those receptor sites, and the mapping of the receptor
sites in various parts of the brain and elsewhere in the body. There has also been substantial progress inunderstanding the effects of cannabis on the respiratory system and on various types of cells in the body’s immune
system. Issues such as the link between cannabis use and schizophrenia, and the nature of cannabis dependence,
have been substantially clarified. In contrast, in a number of other areas there has been no fundamental change inunderstanding. All of these matters are developed in greater depth in the body of this report.

WHO/MSA/PSA/97.4
page 2

1.3

Other sources of recent information

The background papers that served as the basis of this summary report contain much more detailed information andmore complete lists of bibliographic references on the individual topics which they cover (see Annex 2 for acomplete list).
In addition, a number of major reviews of the cannabis literature have appeared during the past few years (e.g. Arif
& Westermeyer, 1988; Hall et al., 1994; Kandel, 1993; Mechoulam et al., 1994; Musty et al., 1991; Adams &Martin, 1996), and these have been taken into account when preparing this report. More detail on various issues
discussed in this report can be found in these reviews.

2.

Cannabis and health: some issues about inference

The approach to assessing the health effects of cannabis use followed in this report is the same as has been adopted
by WHO/PSA to assess the health effects of the use of alcohol, tobacco and other psychoactive substances, namelythat a reasonable standard of scientific proof is required to arrive at conclusions about the probable adverse healtheffects of cannabis. Some of these issues involved in assessing whether scientific evidence does indeed constitutecausality are outlined below.
2.1

Making causal inferences

Causal inferences require, among other things, evidence of an association between cannabis use and an adverse
health outcome; evidence that cannabis use preceded the health outcome; evidence that chance is an unlikely
explanation of the association; and the exclusion of plausible alternative explanations of the association.
Reasonable evidence of an association between cannabis use and a health outcome is provided by the observationof such a relationship in case-control, cross-sectional, cohort, or experimental studies.
If cannabis use is the cause of an adverse health effect then there should be good evidence that cannabis use precedes
the health effect. The strongest such evidence is provided by an observational cohort study or an experiment. In thecase of cannabis, such studies are difficult to conduct due to the fact that cannabis is an internationally controlled
psychoactive substance.
Chance can be ruled out if proper statistical evaluation indicates that the likelihood that the result may have occurredby chance is very small.
The alternative explanation that is much harder to exclude is that any relationship between cannabis use and a healthoutcome is due to an unmeasured variable which causes both cannabis use and the adverse health effect, i.e. a>confounding= factor or factors. Experimental evidence provides the >gold standard= for ruling out such explanations.
This would require the random assignment of persons to use or not use cannabis, so as to ensure that users and nonusers were equivalent in all relevant respects prior to their cannabis exposure. However, such random assignmentis unethical, except in studies of innocuous health effects, because of the unacceptable risks imposed on volunteer
subjects, quite apart from legal considerations in the case of cannabis.
Experiments using laboratory animals permit random assignment of subjects to cannabis or placebo exposure.
There may be, however, considerable problems in extrapolating results across species. These may be minimized byproper attention to the importance of different routes of administration (e.g. oral, intravenous), different forms ofcannabis (e.g. pure cannabinoids versus smoked cannabis plant material), and the question of equivalence of doses
in different species (e.g. rat versus human).

WHO/MSA/PSA/97.4
page 3

When an appropriate animal model does not exist, or when human experiments are unethical, observational studies
are necessary and provided they are properly conducted, non-causal factors can be controlled. If the relationshippersists after such statistical adjustment, then the probability is increased that the health outcome is due to the effectof the exposure, in this case cannabis use.
Causal inferences can be drawn from research findings by judging the extent to which the evidence meets widely
accepted criteria. These include: strength of association, consistency of association, specificity, dose-response,
biological plausibility, and coherence with other knowledge. These criteria are not sufficient to show that an
association is causal but the more that are met, the more likely it is that the association is causal.
2.2

Acute and chronic health effects

Any attempt to summarize the health effects of cannabis, or of any other psychoactive substance, runs the risk ofoversimplification. The health effects experienced by a user will depend not only on the fact that cannabis was used,
but also on a host of other factors. Acute drug effects, for example, will be influenced by the dose, the mode ofadministration, the user’s prior experience with the drug, concurrent drug use, and the user’s expectation, mood state
and attitudes towards substance use, as well as environmental, biological and genetic factors.
The acute health effects of any psychoactive substance are conceptually easier to appraise than its chronic healtheffects: the temporal order of substance use and effect is clear; drug use and its effects typically occur closely
together in time; and if the effects are not life-threatening or otherwise dangerous, they can be reliably reproduced
by administering the psychoactive substance experimentally under controlled conditions. However, in such studies,
the possibility of controlling for blood levels of THC (-9- tetrahydrocannabinol, the main active principle ofcannabis) would allow stronger causal inferences between effects and THC levels as there is a great variability inbioavailability (amount of a substance which is available after absorption by any route) according to the route ofadministration. It is more difficult to attribute relatively rare acute adverse experiences (e.g. flashbacks, psychotic
symptoms) to substance use. It is difficult to decide whether these are: rare events that are coincidental with
substance intake; the effects of other psychoactive substances which are often taken together with cannabis; rare
consequences of substance use that only occur at very high doses; manifestations of unusual forms of personalvulnerability; or the results of interactions between different substances.
Causal inferences about the long-term effects of chronic cannabis use become more difficult the longer the interval
between use and the occurrence of the ill effects; the longer the interval, the more numerous the alternative
explanations that need to be excluded. With continued chronic use this interval does not exist although it is still
difficult to make causal inferences due to concurrent factors. The most rigorous evidence of chronic health effects
is provided by laboratory studies of experimental animals in which well controlled doses are administered over a
substantial period of the animals lives. However, a great many assumptions have to be made in extrapolating fromhealth effects observed in laboratory animals to the probable health effects of equivalent doses and patterns of use
in humans. In addition, there may be problems in extrapolating from studies with pure THC to human experience
with crude cannabis preparations. The plant material contains many other compounds, both cannabinoid and noncannabinoid in nature, and the possibility must always be considered that differences between experimental andclinical observations may be due in part to the effects of these other substances. Ideally, as bioavailability following
the smoking route varies considerably between and within subjects, the measurement of blood levels of
tetrahydrocannabinols should be included in any study design.
Epidemiological studies of the relationship between cannabis use and disease in humans are clearly relevant for
public health policy, but they are less rigorous in assessing the degree of exposure to cannabis and in excluding
alternative explanations of observed associations. There is consequently uncertainty about the interpretation of both
>positive= and >negative= human epidemiological evidence. In the case of positive findings, cannabis use is often
correlated with the use of other psychoactive substances (e.g. alcohol and tobacco) which are known to affect healthadversely. This makes it difficult to confidently attribute (or exclude) some of these adverse health effects tocannabis use. When epidemiological studies fail to find adverse health effects of chronic cannabis use, it isWHO/MSA/PSA/97.4
page 4

nonetheless uncertain whether the substance has indeed few, if any, chronic effects in humans, or whether we havenot used sufficiently sensitive methods or procedures (e.g. cohort size) to reliably detect such effects. Studies on
cannabis-related impairments conducted in cultures with traditional, social use of cannabis, might be helpful indistinguishing between the effects of cannabis use per se and those of a lifestyle often associated with illicit
substance use.

3.

Epidemiology of cannabis use

3.1

Methodological aspects of assessing cannabis use

The earlier literature on patterns of cannabis use is largely based on studies in developed countries, reflecting the
emergence of widespread cannabis use among adolescents and young adults in these countries, and the health, legal
and social concerns that this has led to. However, an increasing number of studies (for example, Smart et al., 1980;
Carlini et al., 1990; Adelekan, 1989; Kramer, 1990) have been carried out in developing countries (in this case,
Bahamas, Brazil, Nigeria and Venezuela, respectively), which provide some insight into cannabis use in developing
countries as well.
Because cannabis is an illegal psychoactive substance, (cannabis and cannabis resin and extracts and tinctures ofcannabis are included in Schedule I of the Single Convention on Narcotic Drugs, 1961, as amended by the 1972
Protocol), data on the levels and patterns of its use are much less widely available than on the use of alcohol andtobacco. Moreover, the illegality of cannabis gives rise to a number of potential biases that operate to underestimate
the prevalence of its use. First, illicit substance users are likely to be under-sampled in household surveys, and those
who are contacted may be reluctant to participate in a survey. Second, even if users agree to participate they may
be less inclined to give truthful responses. Despite these biases there is sufficient evidence for the validity of selfreported substance-use in carefully designed studies to permit inferences about trends in illicit substance use.
No attempt has been made to estimate global figures on the prevalence of cannabis use, given the different methodsof data collection, analysis, definitions and periods when surveys have been carried out. However, the United
Nations Drug Control Programme has estimated the number of cannabis >abusers= (annual prevalence) in the 1990s
at 141 million people, i.e. 2.45 per cent of the world=s population, based on figures given by Member States(UNDCP, 1997).
3.2

North America

3.2.1

United States of America

The United States have regularly undertaken surveys of illicit substance use over the last 15 to 20 years; theseinclude the National Household Survey on Drug Abuse, conducted throughout the US on a regular basis by the
National Institute on Drug Abuse (NIDA) since 1972 (NIDA, 1992); and the >Monitoring the Future= nationwide
surveys of samples of high school seniors, college students and young adults, conducted annually since 1975
(Johnston et al., 1997).
The NIDA national household survey originally covered approximately 9000 persons aged 12 years and older in
randomly selected households throughout the US every two to three years. Since 1991, the survey has been
conducted annually on over 30 000 participants.
In 1992, one-third of the national household sample reported that they had tried cannabis, 9 per cent had used it in
the past year, and 4 per cent reported that they were current users. Lifetime use ranged from 11 per cent among those
aged 12 to 17 years to 59 per cent among those aged 26 to 34 years, while 25 per cent of those over the age of 35
years reported some use. Rates of discontinuation of use were high, with over two-thirds of men and three-quarters

WHO/MSA/PSA/97.4
page 5of women who were ever users not having used cannabis in the last year. Weekly cannabis use was uncommon: it
was higher among men than women (9 per cent of men and 6 per cent of women), with a peak prevalence of 21 percent among those aged 12 to 17 years who had ever used cannabis. Time series data from 1974 to 1990 showed thatthe prevalence of cannabis use increased throughout the 1970s, peaked in 1979, and declined steadily throughout
the 1980s to levels lower than those reported in 1974.
The >Monitoring the Future= surveys (Johnston et al., 1997) show wide fluctuations since 1975 in cannabis use
among American adolescents in secondary school; lifetime prevalence among twelfth-graders (ages 16 to 18 years)
peaked at 65 per cent in 1980 and had fallen by nearly half by the early 1990s; use in the last year peaked at 51 percent in 1979 and fell by more than 60 per cent by 1992. The rate of noncontinuation also rose considerably(Table 1). While most users of other illicit substances had also used cannabis, trends in the use of those substanceswere independent of trends in cannabis use.
Table 1. Trends in Cannabis Use among Twelfth-Graders in the USA% Any Lifetime % Any Use in
Use
Past 12 Months% Noncontinuation Rates*
Used Any

Used 10+
Times

1975

47

40

15

4

1980

60

49

19

5

1985

54

41

25

8

1990

41

27

34

12

1992

33

22

33

11

1993

35

26

26

8

1994

38

31

1995

42

35

1996

45

36

Source: Johnston et al., 1997*

This variable is defined as the percentages of those who ever used the drug (or used it 10 or more times) but did not use it in thepast year. To avoid the implication of confirmed permanent cessation of use, which logically cannot be inferred from the survey,
the investigators used the term >noncontinuation= rather than >discontinuation=.

After more than a decade of steady decline in cannabis use, the 1992 survey of eighth-graders (ages 14 to 16)
and the 1993 and 1994 surveys of eighth-, tenth- and twelfth-graders showed an abrupt rise in all three grades,
and a smaller rise among college students and young adults. There were increases in both the initiation rate andprevalence of continued use (Johnston et al., 1997).

WHO/MSA/PSA/97.4
page 6

Johnston and his colleagues have marshalled considerable evidence that changes in attitudes and beliefs about
cannabis use explained the steady decrease during the 1980s, and the more recent increase in use. They report thatan increased perception of the risks of cannabis use was strongly correlated with decreased rates of use over time,
while in the most recent years, there was a significant decline in perceived dangers of cannabis use (Johnston, 1995).
3.2.2

CanadaA national telephone survey was conducted in Canada in 1994 by Health Canada of 12 155 persons aged 15 years
and older. Overall, 28.2 per cent of the sample reported that they had ever used cannabis, but only 7.4 per cent has
used cannabis in the past year. Rates of use were twice as high among males than females. Prevalence of current
use declined with age from a high of 26.1 per cent among those aged 15 to 17 years to 1.4 per cent among those aged
45 to 54 years and 0.7 per cent among those aged 55 to 64 years. Rates of discontinuation were substantial, with
only 26 per cent of those who had ever used cannabis having done so in the past year.
There have been a number of school surveys conducted in various provinces throughout Canada since the mid-1970s.
The most consistent trend has been an increase in prevalence through the 1970s, then a sharp decline through the
1980s. Rates of illicit drug use were lower in Ontario than in the neighbouring United States of America. The sizeof the decline in rates of annual cannabis use was greater than for other substances. Among cannabis users,
frequency of use has also declined since 1979 (Adlaf et al., 1995). However, there have been some more recent
increases in overall rates of cannabis use within the last few years.
3.3

Australia and New ZealandCannabis continues to be the most widely used illicit psychoactive substance in Australia, with approximately one
third of adults reporting that they had used the substance, and 72 per cent of all young adults between the ages of
20 and 24 at some point in their lives having used cannabis (de Zwart et al., 1994). In a national household surveyof adults conducted in 1993, the prevalence of use was related to gender and age. Men were more likely to have usedthan women, and adults over the age of 45 years were much less likely to have used than younger adults. This
reflects the initiation of widespread cannabis use among young Australian adults in the early 1970s (Donnelly &Hall, 1994).
More than half of those who had ever used cannabis in Australia had discontinued their use, or continued to use lessthan weekly. Seven per cent of women and 15 per cent of men became weekly users. Weekly cannabis use was mostcommon among younger age groups, and highest among those aged 20 to 24 years, declining steeply thereafter. The
use of cannabis has increased dramatically over the past 20 years, with the proportion reporting ever having used
the substance increasing from 12 per cent of adults in 1973, to 28 per cent in 1985, and then to 34 per cent in 1993

WHO/MSA/PSA/97.4
page 7(Donelly & Hall, 1994). The >Omnibus= household surveys that were conducted throughout the 1970s by market
research companies also showed that there has been an increase in the prevalence of cannabis use for all age groups
between 1973 and 1984 (McAllister et al., 1991).
In New Zealand, a 1990 survey showed that 12 per cent of New Zealanders between 15 and 45 years of age had usedcannabis in the previous 12 months (Black & Casswell, 1991) . Forty-three per cent of New Zealanders in the agegroup report lifetime cannabis use. In 1991, a study of 949, 13 to 14 year old children showed that by the age of 15
years, approximately 10 per cent had used cannabis on one or more occasions, and 2.2 per cent reported using
cannabis on more than ten occasions (Fergusson et al., 1993).
3.4

EuropeThe WHO Regional Office for Europe has collected results from a series of surveys in the European region on drug
use among the general population. In 21 countries that were surveyed in the western part of the WHO European
Region, cannabis has been reported as the most used illicit drug in the general population. In Denmark, in 1994,
approximately 37 per cent of those between the ages of 16 and 44 have indicated they had used cannabis at least
once. Using survey data through the mail, approximately 40 per cent of the same cohort had said they had usedcannabis, indicating a slight rise in use in Denmark. In 1991 in Switzerland, 17 per cent of adults aged 17 to 45
reported ever use of cannabis. In a 1994 survey, 50 per cent of youth by the age of 20 said they had tried cannabis
at least once, a considerable increase. However, this may be due in part to the young age of the cohort. In
Germany, a 1994 survey of 18 to 59 year olds indicated that 13.6 per cent of those in the western part of the country
had ever used cannabis, as opposed to 2.8 per cent of those in the eastern part of the country. The prevalence of everuse in the United Kingdom in 1992 was 14 per cent among 12 to 59 year olds (Harkin et al., 1997).
For all of the countries participating in the survey, current use, as expected, was much lower than ever use, indicatingthat cessation of use is very common. In the broad adult population, no more than 6 per cent in all of the countries
for which data are available were using cannabis in the month prior to the survey. Among those in the age group
16 to 19 years, current use is generally higher. In the UK in 1991, 23 per cent of males and 13 per cent of females
in this age group had used cannabis in the previous year, while among the 20 to 24 year age group, 18 per cent ofmales and 11 per cent of females had used cannabis during the last year. In Denmark, two 1994 surveys found that
14 per cent and 10 per cent of those aged 16 to 19, and 12 per cent, and 20 per cent of those 20 to 24 years of age,
had used cannabis during the previous year (Harkin et al., 1997).
Seven of the eight Western European countries which participated in a survey on trends in cannabis use indicatedthat current use was increasing. These include France, where drug use generally has increased among young people
since 1978, and the UK, where in school surveys the percentages of 13 and 14 year-olds who have ever used cannabis
more than doubled from 1989 to 1993; and the percentage at ages 15 and 16 increased sixfold over the same period.
Luxembourg reported that cannabis use had increased by about 20 to 25 per cent in the last ten years (Harkin et al.,1997).
The Pompidou Group (Johnston et al., 1994) has undertaken a study of the feasibility and validity of using highschool surveys to monitor illicit substance use among high school students in Belgium, France, Greece, Italy, theNetherlands, Portugal and Sweden (using a sample from the USA for comparison). The study showed that it waspossible to obtain valid data on illicit substance use. It found that the prevalence rates of almost all illicit
psychoactive substances were generally higher in the USA sample. In the European samples, cannabis had been used
at least once by 10 per cent to 36 per cent of the older student population, and had been used in the past 30 days by
between 3 per cent and 14 per cent of the European students as against 19 per cent of the USA students. Cannabis
was used on a near daily basis by 1 per cent or less of the European samples compared to 3 per cent in the USA.

In the Netherlands, a large national survey of substance use among more than 10 000 high school students aged 10
to 18 years old was conducted in 1992. About one-third of males and one-fifth of females had ever used cannabisWHO/MSA/PSA/97.4
page 8(de Zwart et al., 1994). Data from three national school surveys in 1984, 1988 and 1992, showed large increases
in use between 1988 and 1992, particularly among older males. In the UK, a survey of 3258 randomly selectedhousehold residents found that the lifetime prevalence of illicit substance use was 6.9 per cent, with the mostcommonly used substance being cannabis (Russell et al., 1994).
Taken together, these data suggest that there has been an increase in the prevalence of cannabis use by young people
over the last decade in Europe, as well as in Australia, Canada and the USA.
3.5

Cannabis use in other regions

For over ten centuries, opium and cannabis have been used as therapeutic agents in the treatment of a wide varietyof ailments, or as part of celebrations or as a spice in food in African and Asian countries (UNDCP, 1997). There
are, however limited survey data on trends in cannabis and other illicit substance use in these countries and otherparts of the world. Occasional surveys are reported from specific countries but in most cases these data only provide
a crude indication of levels of cannabis use. Survey methods are rarely reported, and results are often published onlyin summary form. It is often unclear whether reported rates of use refer to ever having used or to more frequent use,
and there are rarely any data on rates of use in different age groups and for men and women. Often only an overall
rate of cannabis use is reported for all adults which understates rates of cannabis use among young adults who are
the heaviest users.
The lack of standardized methods for sampling and surveys, the limited data reported, and the absence of detailed
age and sex-specific rates from most of these countries indicates the urgent need for promoting comparable datacollection among countries. In this respect, WHO has recently completed guidelines on the use of standardized
survey methods for use by countries to promote comparable data collection and to more accurately assess cross1
cultural differences in rates and patterns of cannabis use. With these caveats, the data are briefly described below.
.

3.5.1

AfricaAfrican tribes also made considerable use of cannabis - in Tanzania, the drug found its way into the diet of theSouthern Highlands where cannabis leaves and seeds were used as a spice in the preparation of special dishes.
Traditional healers in Tanzania have also been known to use an extract from the cannabis plant for the treatment of
ear-ache (Kilonzo & Kaaya, 1994).
Cannabis is a traditional psychoactive substance in sub-Saharan Africa, mainly used for ritualistic or medical
purposes, but to a varying degree also accepted as an intoxicant (DuToit, 1980). A few studies on psychoactive
substance use among secondary school students have been conducted in some sub-Saharan countries, mainly in
Nigeria and Kenya (Adelekan, 1989; Dhadphale et al., 1982), showing the existence of experimentation and use ofvarious substances among adolescents, although prevalence was generally lower than in Europe.
In 1990 and 1994, two large surveys were conducted among secondary school students in Zimbabwe (Eide & Acuda,
1995; 1996). In 1990, the prevalence of lifetime use of cannabis among 12 to 14 year old students varied between
5.5 per cent for boys and 1.0 per cent for girls, while among 17 to 18 year old students it varied between 12.7 per

1

WHO Manual on Substance Abuse Epidemiology (in draft). Available from the WHO
Programme on Substance Abuse, Geneva 1997.

WHO/MSA/PSA/97.4
page 9cent for boys and 3.2 per cent for girls. Cannabis use was more prevalent among males of lower socioeconomic
strata living in high-density urban areas, possibly reflecting its comparatively low price.
The data from 1994 indicated a trend towards an increase in lifetime use of cannabis among private school students,suggesting a diffusion from lower to higher socioeconomic strata. This trend needs to be further investigated.
Cannabis use during the last 30 days also slightly increased among male students in the 1994 survey.
Low rates of ever having used cannabis have been reported in small surveys in Namibia and Nigeria. A 1991 surveyof 600 Namibian school children and their parents reported that 8.2 per cent of parents had ever used cannabis, and3.3 per cent were daily users. Among the school children 7.0 per cent had ever used cannabis, 3.7 per cent wereoccasional users and 0.7 per cent were daily users. In Ilorin, Nigeria, a self-administered survey of 1041 secondary
school students in 1988, indicated that use of cigarettes and cannabis occurred significantly more in males, while
no sex differences were noted for any other psychoactive substance surveyed. In Lagos State, Nigeria, a 1991 survey
reported that 5 per cent of the sample had ever used cannabis.
In South Africa, a survey was conducted in 1990 of over 3000 individuals aged 14 years or older on their current
use of psychoactive substances. It was found that 13 per cent of all adult males in urban areas reported current use
of cannabis, compared to 9 per cent in towns, 22 per cent in squatter communities and 5 per cent for tribal areas(Rocha-Silva, 1991). An epidemiological study carried out in 1991 of 7340 students from 16 different schools in
the Cape Peninsula reported that 7.5 per cent of the total sample had ever smoked cannabis. More males thanfemales had ever smoked cannabis, and there was a trend of increased lifetime use with increased grade level inschool (Flisher, 1993). Another study focusing on African youth in both rural and urban areas, aged 10 to 21 years
old, had reported that cannabis use was predominantly found amongst urban males, with 5.5 per cent indicating thatthey were current users (Rocha-Silva et al., 1996).
3.5.2

Latin America and the Caribbean

In Brazil, three large national school-based surveys conducted in 1987, 1989 and 1993 found that the percentageof the surveyed population who had ever used cannabis was 2.9 per cent in 1987, 3.4 per cent in 1989 and 5.0 per
cent in 1993. The highest prevalence of use was in Brasilia (5.6 per cent in 1987, 4.0 per cent in 1989 and 5.3 percent in 1993) and São Paulo (3.5 per cent in 1987, 4.7 per cent in 1989 and 5.7 per cent in 1993). Although in
1993, Porto Alegre had 8.0 per cent of reported lifetime use of cannabis (Carlini et al., 1990; Galduroz et al., 1994).
A number of surveys of drug use have been conducted in Colombia. Torres de Gelvis & Murelle (1990), reported
the findings of a 1987 survey of 2500 residents in four urban centres in Colombia. Respondents were aged between
12 and 64 years. Two per cent of males and 0.3 per cent of females had used cannabis during the previous year.
Rates were low for all age groups with the highest level of 3 per cent for the 20-24 year age group. In terms oflifetime rates, 10 per cent of males and 3 per cent of females had ever used cannabis.
A 1992 National Household Survey on Drug Abuse in Colombia (Ospina et al., 1993) found that 5.3 per cent of
adults reported having used cannabis at least once (10.4 per cent of males and 1.7 per cent of females). Usage washighest among those aged 18-24, with 1.5 per cent having used during the previous year. Only 0.5 per cent of 12 to
17 year olds had used during the past 12 months.
A Mexican household survey of psychoactive substance use in 1988 among respondents 12 to 65 years of age found
an overall rate of lifetime cannabis use of 3 per cent. Cannabis use was higher among males than females (7.6 percent versus 2.2 per cent) and more prevalent among younger age groups. The highest rates of use were in the NorthWest region where 15.4 per cent of respondents aged 12 to 34 years of age reported ever having used cannabis, 7.9
per cent reported having used during the past year, and 4.0 per cent reported having used during the past month(Centros de Integracion Juvenil, 1992). Cannabis has been the most reported drug of initiation in the past 3 years
in Mexico (Tapia-Conyer et al., 1994).

WHO/MSA/PSA/97.4
page 10

Alfaro-Murillo (1990) reviewed a series of surveys of drug use in Costa Rica conducted over the period of 1983-97.
The most recent and representative of these was a 1987 multi-staged survey of 2700 respondents aged from 14 to
60 years of age. Only 3 per cent of the sample reported having ever used cannabis. The same author reported
summary results from a school survey in 1985 of 818 high school students of whom under 5 per cent reported everuse of cannabis.
A cross national analysis of psychoactive substance use in Latin American and Caribbean countries, including
Bolivia, Colombia, the Dominican Republic, Ecuador, Guatemala, Haiti, Jamaica, Panama, Paraguay, and Peru
(Jutkowitz & Eu, 1994) indicated that Jamaica had the highest lifetime prevalence of cannabis use, at 29 per cent,of all countries studied. In other countries, less than 10 per cent of respondents had ever used cannabis. Lifetime
prevalence levels were similar in most of those countries: 7.3 per cent in Guatemala, 6.5 per cent in Colombia, 6.1
per cent in Panama, and 8.3 per cent in Peru. Paraguay had a lifetime prevalence of 1.4 per cent and the Dominican
Republic 2 per cent.
The same study reported that in all countries except Haiti, men were more likely than women to have ever used andto currently use cannabis. In high prevalence countries such as Guatemala and Panama, cannabis use starts as earlyas alcohol and tobacco (15 and earlier), and a substantial proportion of lifetime users currently use the substance(average 40 per cent). In low prevalence countries such as Haiti and Dominican Republic, cannabis use occurs at
an older age on average (in the twenties) than alcohol and tobacco use (Jutkowitz & Eu, 1994).
Smart & Patterson (1990) reported the findings of surveys of drug use among students and delinquents in the
Bahamas. In student samples aged 11 years and over, 8 per cent had ever used cannabis.
3.5.3

Asia

India has a long tradition of cannabis use from Vedic times as part of various religious traditions. In some Asian
countries cannabis is also added to food as a condiment and used in herbal medicines. The extent of such use has
not been well-documented.
With respect to use for psychoactive effects, only recently have limited survey data been collected on patterns or
trends in cannabis use in some regions. Generalization is difficult because of regional differences in patterns of use.
Surveys in three Northern Indian states in 1989 and 1991 (Indian Council of Medical Research, 1993) found a
lifetime prevalence rate of 3 per cent and a prevalence of current use of 1 per cent, with no evidence of any increase
between 1989 and 1991. In Varanasi, a study of 4326 college students revealed that overall cannabis use among
them was 4.5 per cent. This decline in prevalence, from 10.2 per cent in 1976, was mainly in occasional users andthe proportion of regular users had actually increased, with a significant increase among women students (Reddy etal., 1993). In Southern India, a lifetime prevalence of use of 7 per cent has been reported, with 2.5 per cent current
users. Higher prevalence figures of 10 per cent and 27 per cent have been reported in surveys of students(presumably ever use) (Indian Council of Medical Research, 1993).
Household surveys conducted in a rural area of India, an urban slum area and in a city among persons aged 10 years
and older found the following prevalence of ever use of cannabis: 3.2 per cent in the rural area; 3.2 per cent in theslum area; and 2.7 per cent in the city (Machado, 1994). Cannabis use in the rural area was predominantly for
religious purposes whereas in the other two regions its use was mainly recreational. Cannabis use was not perceived
as a problem behaviour in the rural area given the socioreligious context of use, but it was seen as such in the urban
areas where it was perceived as a deviant form of behaviour (Machado, 1994).
No survey results were available to WHO on cannabis use in any other countries from the Asian region.

4.

Chemistry and pharmacology

WHO/MSA/PSA/97.4
page 11

4.1

Terminology

Cannabis is a generic term used to denote the several psychoactive preparations of the plant Cannabis sativa. The
major psychoactive constituent in cannabis is -9-tetrahydrocannabinol (THC). Compounds which are structurally
similar to THC are referred to as cannabinoids. In addition, a number of recently identified compounds that differ
structurally from cannabinoids nevertheless share many of their pharmacological properties. The Mexican term
>marijuana= is frequently used in referring to cannabis leaves or other crude plant material in many countries. The
unpollinated female plants are referred to as sinsemilla. The resin from the flowering tops of cannabis plants iscalled hashish. Cannabis oil (hashish oil) is a concentrate of cannabinoids obtained by solvent extraction of thecrude plant material or of the resin.
4.2

Cannabis and various preparations

Cannabis contains at least 60 cannabinoids, several of which are biologically active. The primary compound ofinterest is (-)-trans-¶-9-tetrahydrocannabinol (hereafter referred to only as THC, unless otherwise specified) whichis the most potent cannabinoid in the plant. Cannabinoids also occur in the plant in the form of carboxylic acidderivatives, e.g. tetrahydrocannabinolic acid. The THC content and the cannabinoid composition are known to vary
widely depending upon the variety and growing conditions. The THC content in cannabis is typically in the rangeof 0.5 to 4 per cent (Huestis et al., 1992). Cannabis oil, hashish and sinsemilla all contain concentrations of THC
exceeding that in the average plant material. Sinsemilla may have THC concentrations of 7 to 14 per cent. THC
content in hashish generally ranges from 2-8 per cent, although it may be as high as 10 to 20 per cent. The
concentration of THC in cannabis oil varies between 15 to 50 per cent. Concerns regarding THC content incannabis have been renewed because of recent developments in indoor hydroponic cultivation techniques. For
example, these efforts have enhanced the THC content in Dutch hemp, so-called >Netherweed=, to concentrations
as high as 20 per cent .
DosageA typical joint contains between 0.5 and 1.0 g of cannabis plant matter which may vary in THC content between 5
and 150 mg (i.e. typically between 1 per cent and 15 per cent). The actual amount of THC delivered in the smoke
has been estimated at 20 to 70 per cent, the rest being lost through combustion or sidestream smoke. The
bioavailability of THC (the fraction of THC in the cigarette which reaches the bloodstream) from marijuana
cigarettes in human subjects has been reported from 5 per cent to 24 per cent. Given all of these variables, the actual
dose of THC absorbed when smoked is not easily quantified.
In general, only a small amount of cannabis (e.g. 2 to 3 mg of available THC) is required to produce a brief
pleasurable high for the occasional user, and a single joint may be sufficient for two or three individuals. A heavy
smoker may consume five or more joints per day, while heavy users in Jamaica, for example, may consume up to
420 mg THC per day. In clinical trials designed to assess the therapeutic potential of THC, single doses have ranged
up to 20 mg in capsule form. In human experimental research, THC doses of 10, 20 and 25 mg have been
administered as slow, medium and high doses.
The pharmacological and toxicological consequences of cannabis exposure are likely due to numerous constituentsin the plant. In addition, smoking cannabis results in the formation of a large number of pyrolysis products. The
vapour phase consists of nitrogen oxides, carbon monoxide, hydrogen cyanide and nitrosamines, and the particulate
phase contains many known carcinogens including phenols, cresols and polynuclear aromatic hydrocarbons. Duringsmoking, tetrahydrocannabinolic acid, which lacks psychoactivity, is converted to THC, thus adding to cannabis
potency.
4.3

Synthetic cannabinoids

In order to investigate the mechanism by which cannabinoids produce their effects in the central nervous system,
numerous structural changes have been made in the THC molecule. Minor structural changes to THC profoundlyWHO/MSA/PSA/97.4
page 12alter its psychoactive potency; this indicates a highly specific mechanism of action. Furthermore, systematic
structural modifications have resulted in the development of novel cannabinoids that are considerably more potent
2than THC. Notable examples include the bicyclic analogue CP 55,940 and 11-hydroxy-dimethylheptyl-¶-8-THC,
the latter being several hundredfold more potent than THC itself. In addition to enhancement of potency, thesesyntheses led to the development of some cannabinoids with structures distinctly different from that of THC. Despite
the development of structural diversity and enhanced potency, all of these analogues produce the full complementof THC behavioural effects.
.

4.4

Receptors

Recognition of the structural requirements for the cannabinoid analogues prompted the search for a cannabinoid
receptor in the brain (Howlett et al., 1990). Using a radio labelled form of a potent bicyclic cannabinoid, Devane
et al. (1988) were able to characterize a binding site that exhibited three key features: cannabinoid agonists boundwith high affinity, non-cannabinoids did not bind, and that this site was present in very high abundance in the brain.
Moreover, Compton et al. (1993) demonstrated a very high correlation between receptor affinity and biologicalpotency of the various compounds, a necessary criterion for a receptor-based mechanism of action.
This receptor is distributed differentially in the various regions of the brain, in a pattern that is similar throughout
a variety of mammalian species, including humans. Most of the receptors are in the basal ganglia, cerebellum,
cerebral cortex and hippocampus. A rough correlation appears to exist between this distribution and some of theeffects of cannabis. For example, binding sites in the hippocampus and cortex may be linked to the subtle effectsof cannabis on cognitive function, while those in the basal ganglia and cerebellum may be associated with cannabisproduced ataxia.
In addition to the cannabinoid receptor in the brain, a peripheral receptor has been identified in macrophages in the
spleen. The peripheral receptor is structurally different from the brain receptor. This observation is importantbecause it suggests the possibility that other receptor subtypes with entirely unique functional roles may exist.
4.5

Endogenous ligands and an endogenous cannabinoid system

An endogenous ligand for the cannabinoid receptor has recently been identified (Devane et al., 1988, and 1992).
Known as anandamide, it appears to have actions which are similar to those of cannabis in several pharmacologicalassays, although it is considerably less potent than ¶-9-THC and has a shorter duration of action.
In order for anandamide to act as a neurotransmitter or neuromodulator, there must be appropriate synthetic andmetabolic pathways. Synthesis has been demonstrated to occur in homogenates of rat brain. Anandamide can bedegraded by a variety of tissues including brain, liver, kidney and lung. Evidence is emerging that a family ofanandamide-type compounds may exist.
Although the recent progress in neuroscience suggests the existence of a cannabinoid neurochemical system, its rolein the brain and its relationship to other neurochemical systems remain to be clarified (Mechoulam et al., 1994).
There is so far no direct evidence for a primary functional role; it may be, therefore, that the endogenous cannabinoid
system is largely neuromodulatory in function. The consequences of manipulating the cannabinoid system can be
only conjectural at this time. Based upon receptor localization in the brain and the pharmacological effects ofcannabinoids, it is reasonable to foresee roles in cognition, memory, reward, pain perception and motor coordination,
2

The chemical name of this compound is (-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3hydroxypropyl)-cyclohexanol.

WHO/MSA/PSA/97.4
page 13

to name a few. At present it remains to be determined whether, and to what extent, use of cannabis will alter
processes which regulate the endogenous cannabinoid system.
4.6

Antagonist

An experimental compound which acts as an antagonist on the brain cannabinoid receptor has been identified(Rinaldi-Carmona et al., 1994). This compound prevents or reverses the pharmacological effects of cannabinoids.
It appears to be highly selective for the cannabinoid receptor, and does not bind to a variety of other brain receptors.
The discovery of this antagonist provides a valuable tool for investigating the functional role of the cannabinoids
in the central nervous system.
4.7

Pharmacokinetics

THC is absorbed more quickly when delivered by smoking than after oral ingestion. Each puff represents a smallbolus of the drug that is delivered to the circulatory system via the capillary bed surrounding the alveolar sacs of thelungs. Huestis et al. (1992) reported measuring detectable amounts of THC (7 to 18 mg/ml) following a single puffof cannabis smoke in individuals smoking cannabis cigarettes (1.75 per cent and 3.55 per cent THC content). Whenexperienced users smoked cannabis cigarettes containing 1.32, 1.97 and 2.54 per cent THC, peak concentrations
developed in excess of 100 mg/ml (Ohlsson et al., 1980; Perez-Reyes et al., 1982; Huestis et al., 1992), although
there was considerable intersubject variability. Obviously, the dynamics of smoking substantially influences how
much of the drug is absorbed. The number of puffs, spacing, hold time, and lung capacity, contribute to thisvariance.
When cannabis is smoked by non-tolerant individuals, physiological and behavioural effects appear rapidly. Huestis
et al. (1992) found that peak effects occurred at 17.4 + 4.8 and 13.8 + 4.2 minutes after initiation of smoking of alow (1.75 per cent) or high (3.55 per cent) dose cigarette. Maximum effects were recorded within 4 to 6 minutes after
the last puff of cannabis smoke.
Plasma levels of THC fall rapidly as the drug redistributes into fatty tissue in accordance with its high lipid
solubility. The delay between peak blood concentrations and peak drug effects are likely related to delays inpenetration in the central nervous system, and to subsequent redistribution of THC following rapid uptake by adipose
tissues (Barnett et al., 1982; Barnett et al., 1985).
Generally, behavioural and physiological effects return to baseline levels 4 to 6 hours after usage. Bloodconcentrations of THC peak prior to drug-induced effects, leading to a dissociation between blood concentrationsof THC and pharmacological effects. This time discordance has led investigators to improve the technology formeasuring THC and its metabolites in biological fluids and tissues (Cook, 1986; King et al., 1987; Gjerde, 1991),
and to develop pharmacokinetic/pharmacodynamic models that establish a relationship between concentrations of
THC and the physiological, behavioural and performance changes produced by cannabis (Chiang & Barnett, 1984).
4.8

Research gaps

It will be important to identify the various neurochemical processes (such as synthesis, release, inactivation, storage,etc.) which constitute the functioning of an endogenous cannabinoid system, and to discover the physiological rolesof this system. This knowledge will be the basis for understanding whether and how the use of cannabis modifies
the endogenous system. Another priority is the development of a range of potent cannabinoid agonists andantagonists with diverse structural features. These are needed to explore the development of potential therapeuticcannabinoid agents. Continued efforts should be made to clarify the relationship between blood cannabinoid levels
and behavioural effects, and to better understand the pharmacokinetics of chronic use, and fetal metabolism
following in utero exposure.

WHO/MSA/PSA/97.4
page 145.

Effects on the brain and behaviour

5.1

Acute effects on central nervous system functions and behaviour

The acute effects of cannabis use have been recognized for many years, and features such as mild euphoria,
relaxation, increased sociability, heightened sensory perception and increased appetite have been described in earlier
reports. The acute effects of higher doses, including perceptual changes, depersonalization and panic have also been
well described previously (ARF/WHO, 1981). Research conducted since the last WHO report has focused mainlyon quantifiable effects such as those on memory, psychomotor performance and appetite; however, some work has
also been done on the acute psychotropic effects of cannabis. A recent report done by Mathew et al. (1993), showedthat cannabis smoking was associated with significant depersonalization that was maximal 30 minutes after smoking.
Other behavioural changes associated with cannabis intoxication included loss of time sense, sensation of >high=,anxiety, tension and confusion (Mathew et al., 1993).
Recent studies have confirmed and extended earlier findings that cannabis can affect memory in various ways. Free
recall of previously learned items is often impaired when cannabis is present both during the learning and the recall;the major impairment is often reflected by intrusions and novel items. In a study done by Block et al. (1992), theacute effects on human cognition of cannabis were assessed. It was found that cannabis impaired all capabilitiesof learning, including associative processes, and psychomotor performance. The only areas that were not affected
were those of abstraction and vocabulary. It was also found that cannabis use altered associative processes, thereby
making uncommon associations more pervasive (Block et al., 1992). Recall of prose material is generally impairedby cannabis, but its effects upon recall of series of numbers (the digit-span test), recognition, and paired-associatestasks (arbitrary word pairings) have been inconsistent. Generally, material learned in the absence of cannabis canbe recalled even if cannabis is present in the blood. Although the acute effects of cannabis on memory appear to bemodest, the possibility must be considered that chronic use by an adolescent might result in a cumulative
developmental impairment.
A substantial number of recent studies have confirmed that cannabis use consistently increases the consumption offood, especially of high carbohydrate foods. In contrast, cannabis has not shown consistent effects upon the users’
subjective reports of appetite. The reason for this apparent dissociation between appetite and food consumption is
not known (Mattes et al., 1994).
Several studies have shown that cannabis appears to increase the perceived rate of the passage of time. Consistent
with earlier observations, numerous studies in the past ten years have confirmed that cannabis impairs psychomotor
performance in a wide variety of tasks, such as handwriting, tests of motor coordination, divided-attention, digitsymbol substitution, and operant tasks of various types (Solowij et al., 1991). The consistency of results is probably
attributable to improved experimental technique, as reflected by greater attention to the importance of taskcomplexity, standardization of THC administration, studies of dose-effect relationships, and of sharper definitionof acute versus residual effects.
A number of studies have examined the acute effects of alcohol and cannabis on various performance tasks, but theresults have been quite varied. In almost all cases the combination had a greater detrimental effect than that of either
drug alone, but in some cases the effects were fully additive, in other cases incompletely additive, and in a few
instances apparently antagonistic. This is an important area for further research.
3

Cannabis has been shown in a variety of ways to function reliably as a reinforcing agent and the degree ofreinforcement is proportional to the THC content (Gardner & Lewinson, 1991).
.

3

reinforcing agent - substance intake leads to repeated use of the same substanceWHO/MSA/PSA/97.4
page 15

Relatively few studies have examined the effects of cannabis on a variety of social behaviours, including verbal andaggressive behaviours. A somewhat inconsistent pattern of changes has been reported following cannabis use, andthe behavioural effects of cannabis can be influenced by the social context of use.
Finally, residual effects of cannabis administration and effects of cannabis and other licit and illicit drugs are poorly
defined and require more research. Cannabis is often used concurrently with other psychoactive substances. The
resultant effects on human behaviour have received less attention and yet the impact polysubstance use has on healthand safety may be substantial.
5.1.1

Dose-effect relationships

Some investigators have attempted to identify a range of THC concentrations in blood or other body fluids which
can be related to behavioural changes, as can be done for alcohol. For THC the relationship of effect to dose iscomplicated by the wide individual variability observed in responses from subjects undergoing performance testing.
This variability can be due to factors related to dose, mode of administration, physiological and pharmacologicaldifferences, complexity of performance tasks, situational demand during testing, and the prior drug experience ofthe subject.
Once equilibrium between brain and blood concentrations is established (approximately 45 minutes after use), a
linear relationship between blood concentrations and pharmacological effects appears (Chiang & Barnett, 1984).
Recently developed mathematical models may clarify the relationship between THC and metabolite concentrations
in blood on one hand and drug-induced effects on the other, and provide estimates of time elapses since last cannabisuse (Huestis et al., 1992).
5.2

Effects on driving

Behaviours for which the acute effects of cannabis might be expected to be particularly important include theoperation of dangerous machinery and driving a motor vehicle. There is sufficient consistency and coherence inthe evidence from experimental studies and studies of cannabinoid levels among accident victims (Smiley et al.,
1981; Stein et al., 1983; McBay, 1986; Soderstrom et al., 1988) to conclude that there is an increased risk of motor
vehicle accidents among persons who drive when intoxicated with cannabis. Impairment of various performancemeasures related to driving skills has been demonstrated immediately following cannabis use and up to 24 hours
thereafter. Simpson (1986) has also shown that cannabis is present in the blood, indicating use within the last few
hours, in 7 to 10 per cent of samples of persons involved in traffic accidents. Williams et al. (1985) in a study ofyoung male fatalities in California, found a rate of 37 per cent samples positive for cannabis. This risk is magnified
when cannabis is combined with intoxicating doses of alcohol. Simpson (1986) also found that 80 per cent of thetime, when cannabis was present, alcohol was also present in the samples.
The relatively small number of experimental studies carried out since the previous WHO report have confirmed thatcannabis can impair various components of driving behaviour, such as braking time, starting time, and reaction to
red lights or other danger signals. However, persons under the influence of cannabis may perceive that they are
impaired, and where they can compensate, they may do so (Stein et al., 1983; Smiley et al., 1981). For example,
they may tend not to attempt to pass the vehicle ahead, to slow down, and focus their attention on the driving task
when they know a response will be required. However, such compensation is not possible when they are presented
with unexpected events, or where the task requires continuous attention, and hence the risk of accidents remains
higher following cannabis use. The effects on driving behaviour are still present for an hour after smoking, but do
not continue for extended periods at the doses used in these studies.
On the other hand, a study carried out on the carry over effects of cannabis on aircraft pilot performance showed thatcannabis use impaired flight performance at 0.25, 4, 8 and 24 hours after smoking. These results suggest that humanperformance while using complex machinery can be impaired as long as 24 hours after smoking as little as 20 mg
of THC, and that the user may be unaware of the drug=s influence (Leirer et al., 1991).

WHO/MSA/PSA/97.4
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Available data are not yet sufficient to quantify the effects of cannabis use as an accident risk. There is uncertainty
about the role of cannabis in motor vehicle accidents in part because blood levels of cannabinoids not only indicatesrecent use and do not indicate whether a driver or pedestrian was intoxicated with cannabis at the time of an accident(Consensus Development Panel, 1985). In addition, more than 75 per cent of drivers with cannabinoids in theirblood have also been found to be intoxicated with alcohol (Gieringer, 1988; McBay, 1986).
A number of these studies have compared the effects of various doses of alcohol and cannabis. In many respects
the effects of these substances are similar, but in others there appear to be differences (Smiley, 1986). For example,
both drugs impaired accuracy of lane control, and increased reaction time to subsidiary stimuli. However, they
produced different effects on patterns of visual search during simulated driving. The influence of personality
variables on these responses may require further study.
5.3

Long-term effects of cannabis on the central nervous system

5.3.1

Cognitive functions

A number of studies carried out in Costa Rica, Greece, Jamaica and other countries in the 1970s appeared to show
little or no effect of chronic cannabis use on cognitive function in groups of adult subjects. These studies assessed
cognitive functions by neuropsychological tests, many of which were originally developed for use in North America
and may have been inappropriate for use in other cultures. However, more recent studies (Fletcher et al., 1996)
making use of improved test procedures and electrophysiological methods, have shown that the long-term use of
cannabis leads to subtle and selective impairments of cognitive functioning. These include the organization andintegration of complex information involving various mechanisms of attention and memory processes, including
verbal learning, card sorting, auditory attention, tone discrimination, and the filtering out of irrelevant information.
Prolonged use may lead to progressively greater impairment, which may not recover with cessation of use for at least
24 hours (Pope & Yurgelum-Todd, 1995), or 6 weeks (Solowij et al., 1991, Solowij et al., 1995), and which could
potentially affect functioning in daily life.
Not all individuals are equally effected. The basis for individual differences needs to be identified and examined.
There has also been insufficient research to address the impact of long term cannabis use on cognitive functioningin adolescents and young adults, and on age and gender differences.

5.3.2

Brain function and neurotoxicity

The review of effects of long-term cannabis use on cognitive function, referred to above, has revealed the existenceof subtle signs of disturbances of brain function. However, these could be interpreted either as evidence of
persisting intoxication with cannabis (a >withdrawal effect=) or as indications of permanent structural or functional
damage to neurons. Evidence to date does not permit a clear conclusion with respect to these two possibilities.
The early claims of gross anatomical changes in the brains of chronic cannabis users have not been substantiatedby later studies with high resolution computerized tomography, in either humans or primates (Rimbaugh et al., 1980;
Hannerz & Hindmarsh, 1983). On the basis of histological and electron microscopic examinations, however, otheranimal studies have suggested that chronic cannabis use produces morphological changes in synapses as well as
hippocampal neuronal loss (Landfield et al., 1988; Eldridge et al., 1992), although others failed to find any
abnormalities (Slikker et al., 1992). More subtle functional changes may possibly be detected by imaging techniquessuch as positron emission tomography (PET), single photon emission computerized tomography (SPECT), andmagnetic resonance imaging (MRI) as these become more widely applied to the study of chronic cannabis users(Volkow et al., 1991).

WHO/MSA/PSA/97.4
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There have been a few investigations of the long-term effects of cannabis on cerebral regional blood flow (CBF).
Tunving et al. (1986) demonstrated globally reduced resting levels of CBF in chronic heavy users of 10 years
compared to nonuser controls, but no regional flow differences were observed. Levels of CBF returned to normal
values with abstinence. However, this study was flawed in that some subjects were given benzodiazepines, which
are known to lower CBF, prior to the first measurement. Mathew and colleagues (1986) in another study did not
find any differences in CBF levels between users and non-user controls.
Therefore one must still rely largely on the persistence of functional disturbances as a means of exploring possible
cell toxicity. If such functional changes last long after the disappearance of cannabis from the body, they might beinterpreted as evidence of permanent damage. Several studies for example, have shown learning and memory deficits
months after the end of chronic cannabis administration to rats. Similarly other studies have shown receptor downregulation or neurotransmitter changes (Oviedo et al., 1993). However, they did not include actual measurementof residual cannabis levels in brain at the time of testing.
In view of the possible impact of chronic cannabis use at critical stages of development, such as during adolescence,
the re-examination of these questions, with due consideration of the methodological issues, is clearly required.
5.4

Adolescent development

There has been a predictable sequence of initiation into the use of illicit drugs among adolescents throughout the
1970s and into the 1990s in which experimentation with cannabis preceded the use of hallucinogens,benzodiazepines, amphetamines, various sedatives, cocaine, and heroin. Generally, the earlier the age of initiation
into cannabis use, and the greater the involvement with it, the greater the likelihood of progression to the use of otherillicit drugs (Kandel, 1984; Kandel, 1988).
However, the causal role of cannabis in this sequence of illicit drug use remains controversial (Kandel & Johnson,
1992). The hypothesis does not imply that a high proportion of those who experiment with cannabis will go on touse heroin, for example. The overwhelming majority of cannabis users do not use other illicit psychoactive
substances. First, cannabis is largely a behaviour of adolescence and early adulthood (in countries where there is no
traditional use of the substance). Second, a more plausible explanation is that it reflects a combination of theselective recruitment into cannabis use of non-conforming and deviant adolescents who have a propensity to use
illicit drugs, and the socialization of cannabis users within an illicit drug using subculture which increases theopportunity and encouragement to use other illicit drugs (Newcombe & Bentler, 1988; Osgood et al., 1988).
There has been some support for the hypothesis that adolescent use of cannabis impairs educational performancein cross-sectional studies (e.g. Kandel, 1984). Cannabis use appears to increase the risk of high-school drop-out andof job instability in young adulthood, but the apparent strength of these relationships may have been exaggerated
as there was no control over differences between groups before initiation of cannabis use (Newcombe & Bentler,
1988).
There is suggestive evidence that cannabis use has adverse effects upon family formation, mental health, andinvolvement in drug-related crime (Kandel, 1984; Newcombe & Bentler, 1988). In the case of each of theseoutcomes, the apparently strong associations revealed in cross-sectional data are much more modest in
longitudinal studies after statistically controlling for associations between cannabis use and other variables
which predict these adverse outcomes.
5.5

Psychiatric disorders due to cannabis use

The major psychiatric syndromes and behavioural disorders that have been putatively linked to cannabis use are: anamotivational syndrome, a dependence syndrome, cannabis induced psychoses, and the initiation and exacerbationof schizophrenia (Basu et al., 1994).

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5.5.1

Amotivational syndrome and cannabis psychoses

The state of the evidence on amotivational syndrome and cannabis induced psychoses has not substantively changed
since the 1981 WHO report. In both cases, the existence of the hypothesized disorders still depends upon
uncontrolled clinical observations. Although there is reasonable self-report evidence that heavy cannabis use can
impair motivation, an amotivational syndrome has not been clearly defined nor have its central features been clearly
distinguished from the effects of chronic intoxication in chronic heavy cannabis users.
The existence of putative >cannabis psychoses= also largely depends upon clinical observations of individuals
developing acute organic and functional psychotic disorders following heavy cannabis use, with the disorders
remitting within days of abstinence from cannabis. There are also a limited number of case-control studies
comparing the clinical symptoms and course of psychotic disorders in individuals who do and do not havecannabinoids in their urine. The occurrence of an acute organic disorder with a brief and self-limiting course has
met with some agreement, even in more recent studies (Chaudry et al., 1991; Thomas, 1993). However, it remains
true that the phenomenology of >cannabis psychoses= has not been clearly defined nor have these putative disorders
been distinguished from schizophrenia and other psychotic problems that occur among cannabis users (Andreasson
et al., 1989; Mathers & Ghodse, 1992). The confirmation of such disorders needs more research evidence.
5.5.2

Cannabis dependence syndrome

Clinical and epidemiological research has clarified the status of the cannabis dependence syndrome. A reducedemphasis on the importance formerly attached to tolerance and withdrawal symptoms in diagnostic criteria for
dependence has removed a major reason for scepticism about the existence of a cannabis dependence syndrome.
Clinical and epidemiological research using standardised diagnostic criteria has produced good evidence for acannabis dependence syndrome that is characterized by impairment, or loss of control over use of the substance,
cognitive and motivational handicaps which interfere with occupational performance and are due to cannabis use,
and other related problems such as lowered self-esteem and depression particularly in long-term heavy users(Anthony & Helzer, 1991). As with other psychoactive substances, the risk of developing dependence is highest
among those with a history of daily cannabis use. It is estimated that about half of those who use cannabis daily
will become dependent (Anthony & Helzer, 1991). Cannabis dependence treatment programmes are not widespreadand the outcome of the treatment often relies on the substance user having a greater sense of economic security anda lower inclination to drop out of the programme, with those staying in the programme having the greatest successrates of quitting cannabis smoking (Stephens et al., 1993; Roffman et al., 1993). The large discrepancy between
population prevalence estimates and the small numbers of cannabis users seeking treatment suggests that there isa high rate of remission in the absence of treatment, while a lack of motivation to seek treatment and to stop use
cannot be ruled out.
Since tolerance and withdrawal symptoms are still widely regarded as diagnostic criteria of substance dependence,
it is worth noting that there is abundant experimental evidence of tolerance to many of the effects of cannabis. Thereis not yet universal agreement about the production of a withdrawal syndrome. Although a recent study has
demonstrated that long term administration of cannabinoid to rats alters the central nervous system in a manner
similar to that observed with other drugs of abuse and also, induces neuroadaptive processes which correspond toa cannabis withdrawal syndrome (de Fonseca et al., 1997). However, withdrawal signs have been described inanimals treated chronically with cannabinoids and then given one of the new receptor antagonists (Aceto et al., 1995;
Tsou et al., 1995).
5.5.3

Cannabis use and schizophrenia

Epidemiological research has produced clear evidence from case-control, cross-sectional and prospective studies ofan association between cannabis use and schizophrenia. The prospective study of Andreasson et al. (1987), has
shown a dose-response relationship between the frequency with which cannabis had been used by age 18 and therisks over the subsequent 15 years of being diagnosed as schizophrenic. The association is not in doubt but its

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significance remains controversial because it is unclear whether it reflects the precipitation of schizophrenia bycannabis use or the increased use of cannabis and other drugs as a consequence of schizophrenia (Williams et al.,1996). In a more recent study (Allebeck et al., 1993), out of 229 people in Stockholm County with a diagnosis ofcannabis dependence and psychosis, 112 fulfilled DSM III-R criteria for schizophrenia. In most cases, regularcannabis use preceded onset of psychosis by at least one year, which supports the argument that cannabis is a risk
factor for schizophrenia. Further supportive evidence has been provided by the WHO international prospective
study of schizophrenia (Jablensky et al., 1991).
5.5.4

Other disorders

A number of other psychiatric disorders have been linked with cannabis use. These include: an amnesic syndrome,persistent depersonalisation, and flashbacks. There is no evidence to support the existence of an amnesic syndrome
comparable to the Wernicke-Korsakov syndrome that occurs in chronic heavy alcohol users. There is some evidence(reviewed in the chapter on cognitive effects) that chronic heavy cannabis use may produce more subtle cognitive
impairments. The other two disorders are only supported by a small number of case histories and, in the case of
flashbacks, there is considerable uncertainty as to whether these effects were attributable to cannabis or other drug
use.
5.6

Priorities for future research

A better understanding of the effects of chronic cannabis use is required. This includes research which explores theeffects on cognitive functioning, especially during critical developmental phases such as adolescence. In addition,
the question of cannabis-produced brain damage can be answered only by ascertaining if there are morphologicalchanges in the brain that correlate with functional deficits which outlast the duration of cannabis in the brain; thesestudies will require use of newer neuroscience techniques in experimental animals as well as in humans.
The major psychiatric research priorities are: better delineation of the clinical features of cannabis dependence,
including its responsiveness to interventions to assist users to stop; and intervention studies with schizophrenic
individuals who use cannabis, to see whether cessation of such use improves outcomes.

6.

Effects on the respiratory system

Worldwide, after tobacco, cannabis is probably the most commonly smoked substance. With the exception of
nicotine in tobacco and over 60 cannabinoids in cannabis, the smoke from these two compounds share many of thesame respiratory irritants and carcinogens. In fact, the tar phase of the smoke of marijuana has about 50 per centmore of some of the carcinogens than a comparable quantity of unfiltered tobacco (Leutchtenberger, 1983; Instituteof Medicine, 1982).
6.1

Histopathology

Early animal studies indicated that a prolonged high dose exposure to cannabis smoke could result in parenchymal
lung injury (Fleischman et al., 1979; Rosenkrantz & Fleischman, 1979). The findings of later experimental studies
suggest that high dose cannabis exposure is associated with the development of chronic obstructive bronchitis, andcarries the risk of invasive malignancy such as that produced by tobacco smoke. In a longitudinal study done on
the respiratory effects of cannabis, a total of 1802 subjects who smoked cannabis were followed for six years. The
study found an increased risk for >chronic cough= (RR of 1.73) and of >wheezing= (RR of 2.01), after adjusting forage, tobacco smoking and occurrence of the symptom in a preceding survey. There was also a significant reduction
in pulmonary function, with exposure as low as the consumption of less than one cannabis cigarette per day for one
year or more (Sherrill et al., 1991). The histopathological changes occurred mainly in the distal airways andincluded acute and chronic inflammation, fibrosis, and alveolar cell hyperplasia (Rosenkrantz & Fleishman, 1979).

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Later prospective investigations undertaken on primates found changes such as bronchiolar squamous metaplasia,
and peribronchiolar/interstitial fibrosis. The severity of these small airway changes was related to the dose andduration of cannabis exposure with higher doses and longer exposure to smoke leading to greater changes, while
atypical cell hyperplasia with focal atypia were also found (Tashkin et al., 1987).
In human studies, it has been shown that the principal respiratory damage caused by long term cannabis smoking
is an epithelial injury of the trachea and major bronchi. In a study undertaken in Germany, it was found that the
number of T-cell lymphocyte counts were lower in chronic cannabis smokers, thereby increasing their chances ofdeveloping squamous cell carcinoma of the oral cavity and pharynx (Wengen, 1993). The difference between thefindings in animal and human studies is probably due to the fact that observations in humans are limited to thosewhich can be made by bronchoscopy. Human bronchoscopic studies undertaken on young adults who had no
respiratory symptoms found evidence of histological changes in the upper airways among heavy cannabis smokers.
These changes included basal cell hyperplasia, stratification, goblet cell metaplasia, and basement membranethickening (Roby et al., 1991; Tashkin et al., 1987). The few studies that have been undertaken generally show that
there is no additive effect between cannabis and tobacco (Gil et al., 1995; Sherill et al., 1991; Tashkin et al., 1987),
although one study, the Tucson study, did show an additive effect (Bloom et al., 1987). However, in a follow-up
survey the same additive effect was not found (Sherman et al., 1991). Whether or not there is an additive effectbetween tobacco and cannabis smoking remains uncertain, and more research needs to be done to further explore
this issue. The histological abnormalities resulting from cannabis consumption were more severe per cigarettesmoked than for tobacco (Wu et al., 1988).
Autopsies undertaken on cannabis smokers who had no respiratory symptomatology at the time of death also foundchanges in the form of focal infiltration by pigmented macrophages around bronchioles and within alveolar spaces,
and focal fibrosis within alveolar walls. In this study, the relative contribution of tobacco to these changes could
not be ascertained with certainty except in one case though the person did not smoke tobacco (Fligiel et al., 1991).
6.2

Immune defence

The alveolar macrophage, the key cell in the lung=s defence against infection, has been shown to be impaired bycannabis smoke in both animal and human studies. While animal studies failed to demonstrate a change inmacrophage numbers following cannabis smoke exposure, subsequent investigations in humans, comparing nonsmokers to cannabis and tobacco smokers, suggested an increase in the number of macrophages in habitual cannabis
smokers (Wallace et al., 1994; Barbers et al., 1987; Barbers et al., 1991). This probably reflects an immunologicalresponse to any lung injury induced by cannabis smoke. The effect is independent of tobacco consumption (Wallace
et al., 1994).
Macrophage cells harvested in these human studies have alterations to their morphology, possibly reflecting an
impairment in cell function. Residual particles from cannabis smoke in the form of intra cytoplasmic inclusions
have been found to be cycled between subsequent generations of macrophages as part of the process of cellularturnover (Davis et al., 1979). While there is a suggestion in human studies that cannabis smoking did not alter
phagocytosis or respiratory burst, it possibly did impair destruction of ingested organisms (Lopez-Cepero et al.,1986). The mechanism of macrophage impairment has not been fully elucidated and requires further investigation.
These studies suggest that regular cannabis consumption reduces the respiratory immune response to invadingorganisms. Further, serious invasive fungal infections as a result of cannabis contamination have been reported
among individuals who are immunocompromised, including a series of patients who were affected by AIDS
(Denning et al., 1991).
These findings suggest that persistent cannabis consumption over prolonged periods can cause airway injury, lunginflammation, and impaired pulmonary defence against infection. Epidemiological studies that have adjusted forsex, age, race, education, and alcohol consumption suggest that daily cannabis smokers have a slightly elevatedrisk of respiratory illness compared to nonsmokers.

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6.3

Lung physiology

Several studies on humans have demonstrated an acute bronchodilator effect of both smoked cannabis and oral THC.
These findings have been replicated in both healthy and asthmatic populations (Boulougouris et al., 1976).
However, the potential therapeutic use of cannabis and synthetic cannabinoids as bronchodilators has been largely
discounted.
Two recent studies in relatively young populations compared respiratory symptoms and lung function in nonsmokers
and long-term smokers of both cannabis and tobacco (Bloom et al., 1987; Tashkin et al., 1988). In both studies,
heavy habitual cannabis consumption, with or without tobacco, was associated with a higher prevalence of symptomsof chronic bronchitis and a higher incidence of acute bronchitis than in the nonsmoking group.
However, the studies disagreed about effects on peripheral airway resistance. One longitudinal study indicated thatcannabis consumption was associated with increased large airway resistance but not with the development of chronic
obstructive bronco-pulmonary disease or emphysema (Gil et al., 1995; Tashkin et al., 1988). The other study founda significant deleterious effect on the ventilatory function of small airways and alveoli among habitual cannabis
smokers. The effect was at least as great as the effect of tobacco consumption (Bloom et al.,1987). Recent studies
have also failed to agree on whether any impairment in pulmonary function is additive to the effects of tobacco
consumption. Both the site of impairment and potential interaction between cannabis and tobacco require further
investigation.
While the pulmonary absorption of carbon monoxide from cannabis smoke is relatively high compared to that fromtobacco smoke, the impact of this on heavy habitual consumers is modest. This probably reflects the short half-life
for clearance of carbon monoxide, and the relatively longer intervals between occasions of cannabis use. However,the carboxyhaemoglobin levels in cannabis smokers are higher than in nonsmokers; this may result in a slightinterference with tissue oxygenation (Tashkin et al., 1988).
Ventilatory responses to rebreathing carbon dioxide have been shown by different studies to decrease (Bellville etal., 1975), increase (Zwillich et al., 1978), or remain unchanged (Vachon et al., 1973) immediately after smoking
cannabis. A recent, more detailed study of control of breathing responses to smoking cannabis of varying potency
has failed to reveal any acute effect of cannabis on central or peripheral ventilatory drive or on metabolic rate inhabitual cannabis smokers (Wu et al., 1992).
6.4

Carcinogenesis

Cases of cancers of the aerodigestive tract have been reported in young adults with a history of cannabis use (Taylor,1988; Fergusson et al., 1989; Donald, 1991). Attribution of causality has been hindered because many of the cases
also used alcohol and tobacco (Polen et al., 1993). However, these cases are of particular concern because suchcancers are rare in adults under the age of 60, even among those who smoke tobacco and drink alcohol. Case-control
and experimental studies of such cancers should be a high priority for research on the possible adverse health effects
of chronic cannabis use.

7.

Effects on endocrine and reproductive systems

Studies of the effects of cannabis on male and female reproductive systems have examined: the pituitary hormones
luteinizing hormone (LH) and follicle-stimulating hormone (FSH) which are responsible for the synthesis of the sex
hormones (estrogen and testosterone) and the normal functioning of the sex organs; the pituitary hormone prolactin
which alters LH and FSH levels; the levels of estrogen and testosterone; and the physiological status of the sexorgans.

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7.1

Male reproductive hormones

Although there is general agreement that THC treatment lowers plasma levels of LH and testosterone in malelaboratory animals (Symons et al., 1976; Chakravarty et al., 1982; Puder et al., 1985; Fernandez-Ruiz et al., 1992),
the effects in the human male are not as clear. Early studies reported that cannabis exposure produced either atransient reduction (Schaefer et al., 1975; Cohen, 1976) or no effect in levels of plasma LH and testosterone inhuman males (Cushman, 1975; Mendelson et al., 1978). However, the findings that chronic cannabis exposure does
affect human reproductive function (Kolodny et al., 1974; Hembree et al., 1979; Issidorides, 1979) suggests thatcannabinoids do alter the reproductive hormones that control testicular function and have some effect on testicularparameters.
Subsequent studies have largely confirmed that plasma LH levels were unchanged after smoking one to two cannabis
cigarettes containing 2.8 per cent THC (Cone et al., 1986). Daily exposure to either oral THC or cannabis smoking
had no effect on plasma levels of either LH or testosterone in men who were prior cannabis users. Similarly,
circulating levels of LH, FSH, prolactin and testosterone were not found to be clinically different between cannabis
users and non-users (Markinanos & Stefanis, 1982; Dax et al., 1989; Block et al., 1991).
The conflicting results in these human studies could reflect differences in experimental procedures and the possible
effect of previous cannabis exposure (i.e. tolerance) and other drugs in test subjects, effects which can be carefully
controlled in animal studies.

7.2

Female reproductive hormones¶-9-THC alters pituitary secretion of LH, FSH, and prolactin when administered either acutely or repeatedly to both
intact and ovariectomized experimental animals (Steger et al., 1980, 1981). In addition to suppressing normal
circulating levels of LH in female rats, THC also inhibits the surges of LH and FSH which are essential for ovulation.
As a consequence, THC blocks ovulation in intact rats and monkeys (Smith et al., 1979). Moreover, the occurrenceof the first ovulation in the maturing female rat is delayed following peripubertal administration of THC (Field &Tyrey, 1986). Tonic prolactin levels are significantly decreased following acute or repeated THC treatment in both
intact and ovariectomized female rats (Hughes et al., 1981). In addition, prolactin surge occurring under a variety
of situations is blocked by THC administration (Steger et al., 1983). THC treatment also disrupts the normal
rhythm of menstrual cycles in monkeys (Asch et al., 1981).
Cannabis smokers have shorter menstrual cycles due to an inadequate luteal phase. When the acute effects ofcannabis smoking on plasma LH, prolactin and sex steroid hormones were evaluated during the follicular and luteal
phases of the menstrual cycle in groups of adult females, it was found that during the luteal phase of the menstrual
cycle, plasma LH and prolactin were suppressed, thereby shortening their cycles (Mendelson et al., 1985, 1986).
However, self-reported chronic cannabis users were not found to have any changes in circulating levels of LH, FSH
and prolactin when compared to non users (Block et al., 1991). It appears that the hormonal response to cannabis
exposure depends on the stage of the menstrual cycle.
7.3

Target organs

In addition to the aforementioned hormonal alterations, animal studies show that cannabinoid exposure reduces
testicular, seminal vesicle and prostate weights, decreases ovarian weight and increases pituitary and adrenal weights.
These cannabinoid effects have been attributed to actions at both peripheral and brain sites.
7.4

Other hormones

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There is considerable evidence that cannabinoid exposure can affect the hypothalamic-pituitary-adrenal axis. Early
animal studies have demonstrated that THC is a potent stimulator of adrenocorticotropin (ACTH) release in malerats. Acute THC administration also elevates plasma corticosterone levels in male and female rats. Anandamide,
the endogenous ligand for the cannabinoid receptor, produces some effects similar to those of THC.
Landfield et al. (1988) observed that THC administration induced aging-like degenerative changes in rat brain tissue
similar to those resulting from elevated corticosterone. These investigators have also demonstrated that THC
administration alters the corticosterone receptor in brain. It seems possible that the corticoid and anandamide
systems could be mutually regulatory. Additional evidence will be needed if such a relationship is to be established.
However, it should be noted that men who were heavy cannabis users did not exhibit alterations in cortisol level(Block et al., 1991) or impaired adrenocortical reactivity to ACTH.
In early studies, it was shown that THC inhibited growth hormone secretion in male rats. However, other studies
have demonstrated either no change or an increase in growth hormone in male rats treated with THC. More recently
it has been shown that infusion of THC directly into brains of adult male rats suppresses growth hormone secretion.
Although differences in animal models, and route and doses of THC administration may explain these variable
growth hormone responses to THC, more studies are warranted to determine conclusively the effects of cannabis
exposure on growth hormone in both males and females.
Few other endocrine systems have been studied. Circulating thyroxine levels have been shown to be reduced
following acute or chronic THC administration in male rats and rhesus monkeys. THC treatment may also affect therelease of the posterior pituitary hormone oxytocin (Tyrey & Murphy, 1988).
7.5

Implications for hormonal alterations

Early experimental studies reported decreased sexual activity, including copulatory behaviour, in male rats exposed
to THC. To date, controlled studies investigating the effects of cannabis exposure on copulatory behaviour of theadult female have not been reported.
There is very little literature concerning the effects of cannabis on fertility per se. The findings that cannabinoids
interfere with normal hypothalamic-pituitary-gonadal function (Murphy et al., 1990) and can disrupt ovulation andsperm production suggest that fertility could be affected. However, there is no epidemiological evidence on the issue.
Cannabis-induced reductions in testosterone and spermatogenesis observed in some studies are probably of little
consequence in adults. This action of cannabis might be of importance in the prepubertal male or in individuals
whose fertility is already impaired for other reasons; however, at present this is purely conjecture.
7.6

Future directions

Although there is no compelling evidence for an alteration in fertility by cannabinoids, further research is needed
considering the comparatively high prevalence of cannabis use during the early reproductive years. Future researchshould examine interactions between the endogenous cannabinoids and glucocorticoids.

8.

Effects on intrauterine and postnatal development

8.1

Background

Knowledge of how much active substance is delivered to biological tissues is basic to an understanding of thepharmacology and toxicity of a compound. Nearly all of the developmental studies on animals have investigated only
pure THC and have allowed the measurement of amounts of drug actually delivered to the placenta and foetus. In

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contrast, human reproductive studies have typically investigated the effects of smoked cannabis of uncertain THC
content resulting in unknown concentrations of active drug in the fetal-placental unit. These issues complicate theinterpretation of research on the effects of cannabis use on pre- and postnatal development.
8.2

Animal studies

Abel (1985) has pointed out the serious methodological and interpretational flaws that characterized the early
literature of animal studies on the developmental toxicity of THC. Adverse effects observed in offspring may nothave been produced by direct drug effects on the embryo and foetus but may have been secondary to poor maternal
nutrition and dehydration. THC can also disrupt normal maternal care at parturition and, through hormonal effects,
inhibit milk production and release, all having possible adverse consequences for the development of the offspring(Hutchings, 1985).
In studies using appropriate controls for maternal nutrition and fostering, several dose-related effects have beenfound in rat offspring following administration of THC to the pregnant dam. At birth, a dose-related increase in the
male: female ratio of live offspring was consistently found, suggesting that female foetuses have a greater
susceptibility to THC lethality (Hutchings et al., 1987; Morgan et al., 1988). During the postnatal period, a doserelated inhibition of both somatic growth and brain protein synthesis was found. These effects were transitory,
however, and the THC-exposed animals caught up to the controls by the time of weaning (Hutchings et al., 1987).
Hutchings found no evidence of neurobehavioural deficits in the offspring, independent of maternal toxicity; thesefindings are consistent with those of other well-controlled animal studies (Abel, 1985).
8.3

Human studies

8.3.1

Epidemiological studies

Epidemiological studies of the prevalence of drug use in pregnant women are often complicated by sampling
procedures of uncertain validity. Human studies should be based on representative samples of the whole populationof pregnant females. In many studies, however, the samples are selectively biased toward drug users. There is also
uncertain comparability of self-reports of drug use gathered before birth (i.e. prospectively) with those obtained after
birth (i.e. retrospectively). Exact doses and patterns of use are usually difficult to quantify. The effects of otherdrug use and socioeconomic factors are often difficult to assess by case control methods. No objective markers ofcannabis use are yet available, although some new methodologies are being developed to identify and quantify inutero cannabis exposure, i.e. hair analysis, meconium analysis.
Despite these problems there is reasonable evidence that cannabis use during pregnancy impairs fetal development,
leading to a reduction in birth weight, perhaps as a consequence of shorter gestation, and probably by the samemechanism as cigarette smoking, namely, fetal hypoxia. In a case study done on the effects of prenatal cannabis use
on offspring growth from birth through to three years of age, prenatal cannabis exposure was associated only withdecreased length at birth. Neither tobacco nor cannabis use predicted gestational age or morphological abnormalities(Day et al., 1992). Young maternal age also seemed to increase the negative effects from prenatal tobacco andcannabis exposure (Cornelius et al., 1995). Prenatal exposure to cannabis was also associated with disturbed
nocturnal sleep patterns at three years of age, with more arousals and lower sleep efficiency (Dahl et al., 1995).
There were no effects present of prenatal cannabis exposure on growth when the children reached age six (Day etal., 1994a). There is uncertainty about whether cannabis use during pregnancy produces any increase in the risk ofbirth defects by direct toxicity in utero. The limited number of human studies have not consistently shown any
increased rate of birth defects.
There is little evidence that cannabis use can produce chromosomal or genetic abnormalities in either parent which
could be transmitted to their offspring. Such animal and in vitro evidence as exists suggests that the mutagenic
capacities of cannabis smoke are greater than those of THC, and are probably of greater relevance to the risk of users
developing cancer than to the transmission of genetic defects to the offspring. There are a few case-control studies

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which suggest that there is an increased risk of several rare cancers (childhood non-lymphoblastic leukaemia,
rhabdomyosarcoma, and astrocytoma) among children born to women who used cannabis prior to conception or
during pregnancy (Neglia et al., 1991; Robinson et al., 1989, Kuijten et al., 1990). Further exploration of theseclaims is warranted as reporting bias may have been an alternative explanation to the findings.
8.3.2

Neurobehavioural studies

The results of a large-scale prospective study, the Ottawa Prenatal Prospective Study of maternal cannabis use(Fried, 1980; Fried, 1995), suggest that any long-term consequences of prenatal exposure to the child are very
subtle. In the newborn and neonate, there appears to be an association between nervous system function and prenatal
exposure. Between 6 months and 3 years of age, no neurobehavioural consequences of maternal cannabis use were
found (Frief & Watkinson, 1988; and 1990). However, at 4 years, offspring of regular cannabis users showed
reduced verbal ability and memory. Similar deficits were seen in school-aged children as well (Day et al., 1994a;
Fried, 1995), and were accompanied by decreased attention and by increased impulsiveness in the children of thosemothers who had been the heaviest users of cannabis during pregnancy. These results suggest that in utero exposure
to cannabis can affect to some degree the mental development of the growing child (Day et al., 1994b). Given theimportant implications of such a relationship, future research should address this possibility.
Finally, knowledge on fetal metabolism of cannabis and its components is limited, although this would be important
to determine the effects of in utero drug exposure. New methodologies are being developed to identify and quantify
in utero drug exposures, i.e. hair analysis, meconium analysis. More appropriate and objective markers of in utero
cannabis exposure have not yet been developed and the existent drug anolytes (tetrahydrocannabinol and/or 11-nor¶-9-tetrahydrocannabinol) are not adequate to monitor.

9.

Effects on cell nuclei

Most information on interactions of cannabis with cell nuclei deals with four types of effects; (1) those on
macromolecular synthesis, (2) chromosomal aberrations, (3) mutagenicity, and (4) carcinogenicity. In assessing
this evidence it is important to bear in mind that the effects of pure cannabinoids, such as THC, will almost certainly
be different from those of complex mixtures of large numbers of known and unknown compounds that make up
cannabis smoke. Differences in the material examined may account for some of the contradictory findings in theliterature.
Cannabinoids can interfere with the normal cell cycle (Zimmerman & McClean, 1973) and can also decrease
synthesis of DNA, RNA and protein (Blevins & Regan, 1976). More recently, Tahir & Zimmerman (1992) haveshown that THC can disrupt the formation of microtubules and microfilaments in rat cells in culture and hence mayinterfere with such diverse cellular processes as cell division, cell migration and neuron differentiation. Anotherrecent study (Mailleux et al., 1994) showed a significant increase was produced in expression of the gene coding forthe growth factor pleiotropin and could be detected in the adult rat forebrain following a single intra peritoneal
injection of THC (5 mg/kg), a finding which needs further investigation.
Regarding the possibility of cannabinoids to induce chromosome aberrations, including chromosomal breaks,
deletions, and other errors in chromosomal separation, the literature is still inconclusive and has shown contradictory
results (Zimmerman & Zimmerman, 1990-91; Chiesara et al., 1983; Piatti et al., 1989; Behnke & Eyler, 1993).
There is general consensus that both cannabis use (as judged by findings in users or animals exposed to marijuanasmoke) and exposure to certain constituents of cannabis preparations (or in some cases cannabis smoke condensates)
may indeed have mutagenic effects in the Ames assay for mutagenicity (Busch et al., 1979; Wehner et al., 1980;
Sparacino et al., 1990). On the other hand, pure THC as such has been found to have no mutagenic effects(Zimmerman et al., 1978; Generoso et al., 1985; Berryman et al., 1992).

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The demonstrated mutagenicity of cannabis smoke would predict some risk of carcinogenicity. Most of the new
evidence, however, consists of case reports of upper respiratory or oropharyngeal cancers in cannabis smokers(Wengen, 1993) and no full-scale epidemiological studies have been conducted.
Future research in this area should focus on the following areas: systematic comparisons of cannabis smoke
condensates, smoke fractions and pure cannabinoids in the same experimental systems for studying production ofchromosome aberrations, especially in cultured human cells; systematic comparisons of the mutagenicity of cannabis
and tobacco smoke; additional studies on the carcinogenicity of cannabis preparations in a variety of animal models;and epidemiological studies of the risks of cancer related to cannabis smoke as compared to tobacco smoke.

10.

Effects on immune system

Many research studies on cannabis and immune system functions in whole animals and tissue culture systems havebeen published in the past 10 years (e.g. Hollister, 1988; Friedman et al., 1994). Cannabinoids, especially THC,
have been found to modify the function of a variety of immune cells, increasing some responses and decreasingothers. This variation in drug effects depends upon experimental factors such as drug concentration, timing of drugdelivery, and the type of cell function analysed. The range of cell types and functions studied is very broad,including such diverse phenomena as morphology, biochemistry and phagocytic activities of macrophages fromhumans and other species, both in vitro and in vivo; production and release of cytokines, prostaglandins, and othermediators of immune responses; B and T lymphocyte responses in vivo and in vitro; antibody formation; andresistance to infections, in both experimental models and human disease, especially in AIDS patients.
The results of all of these investigations indicate that cannabinoids are immunomodulators, i.e. capable of perturbing
immune system homeostasis when administered to the living subject or when added to cell cultures. However, it
is also clear that the immune system is relatively resistant to these drugs. Many of their effects appear to berelatively small, totally reversible after removal of the cannabinoids, and produced only at concentrations or doses
higher than those required for psychoactivity (more than 10FM in vitro, or more than 5mg/kg in vivo). Moreover,immunomodulatory effects can be produced by some cannabinoids which do not induce psychoactive effects. Thesefindings suggest that cannabinoid effects on immune cells may not be exclusively mediated by the recently describedcannabinoid receptors, even though such receptors have been demonstrated in these cells. However, the existenceof a cannabinoid receptor on macrophages, that differs significantly from the cannabinoid receptor in the brain,suggests that the possible role of receptor-mediated actions in immunomodulation by cannabinoids requires further
study.
Unfortunately, the health impact of any immune effects of cannabis smoking is still unclear. Many studies haveclearly established that THC can act as an immunomodulator. However, relatively few of them have employedanimal paradigms or human subjects in studies designed to test the effects of cannabis exposure on host resistanceto microbes, viruses and tumours. Several animal studies have demonstrated impairment of resistance to bacterial
or viral infections in mice exposed to cannabis smoke or THC, but the results have not been entirely consistent.
Moreover, most studies have used cannabinoid doses that are difficult to relate to levels self-administered by humans.
It is clear that well designed studies of this topic are needed, and that they will require the cooperation of
immunologists, infectious disease specialists, oncologists and pharmacologists in their design, execution andinterpretation.

11.

Effects on other organ systems

11.1

Cardiovascular effects of cannabisWHO/MSA/PSA/97.4
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The most consistent and reproducible of the human effects of cannabis is dose dependent tachycardia, i.e. an increasein heart rate that correlates with the subjective ratings of >high=. Both sympathetic and parasympathetic mechanisms
seem to be involved in this cannabis-induced tachycardia.
Increases in heart rate lead to increases in cardiac output but the extent of the effect on blood pressure depends on
peripheral resistance. The cannabis-induced increase in heart rate may increase cardiac output as much as 30 per
cent, yet increases in supine blood pressure are usually less than 10 per cent. Postural hypotension can be
exaggerated. Cannabis has been shown to increase blood flow in the limbs.
Acute cardiovascular effects of cannabis are less likely in adolescents and young adults, among whom prevalenceof use is highest; however, a few reports of myocardial infarction in young cannabis smokers deserve closer
examination (Choi & Pearl, 1989; Podczeck et al., 1990).
The possibility remains that chronic heavy cannabis smoking may have more subtle effects on the cardiovascularsystem, analogous to the long-term cardiotoxic effects of tobacco smoking, because the cardiovascular effects of
THC and nicotine are similar. Moreover, since many cannabis smokers are also cigarette smokers, there is thepossibility that there may be adverse interactions between nicotine and cannabinoids in their effects on the
cardiovascular system. This requires further research.
The cardiovascular effects of cannabis may be hazardous for patients with hypertension, cerebrovascular disease andcoronary atherosclerosis for whom marijuana poses a threat by increasing the work of the heart. The severity andprevalence of this threat remain to be determined as the cohort of chronic cannabis users of the late 1960s and early
1970s enters the age of maximum risk for various cardiovascular complications.
11.2

Effects of cannabinoids on the liver and gastrointestinal tract

There appears to be little or no human or animal evidence that acute or chronic use of cannabinoids affects liverfunction. There is reasonable animal evidence that cannabinoids decrease intestinal motility and delay gastric
emptying. There is no evidence of significant symptoms of constipation as a consequence, and, as typically used,cannabis has minimal effects on the absorption of alcohol.
The most interesting aspects of the gastrointestinal effects of cannabis are theoretical and therapeutic. The sites ofaction of the antinauseant and antiemetic effects, and also of the stimulant effects of the cannabinoids on foodintake, remain to be identified. As with studies of the opioids, the isolated intestine preparation may serve as a
useful model for the study of cannabinoid receptors, and may provide the opportunity to differentiate between central
and peripheral receptors.

12.

Therapeutic uses

12.1

BackgroundThe broad range of potential therapeutic applications of cannabinoids reflects the wide distribution of cannabinoid
receptors throughout the brain and other parts of the body. The possibility of distinct subtypes of cannabinoid
receptors and the probable development of new compounds to bind selectively to these receptors, as either agonists
or blockers, may well open the door to the selective treatment of a number of disorders. In time, some of thesecompounds may be targeted specifically to one function or another of the endogenous cannabinoid system.
Despite the positive appraisal of the therapeutic potential of cannabinoids as an antiemetic and antiglaucoma agent,
they have not been widely used and the clinical research undertaken is limited. Other therapeutic uses forWHO/MSA/PSA/97.4
page 28

cannabinoids warrant further basic pharmacological and experimental investigation and clinical research into theireffectiveness.
12.2

Utility as an anti-emetic agent in cancer chemotherapy

The moderate efficacy and safety of THC in the control of nausea induced by cancer chemotherapy was establishedin experiments in the late 1970s and early 1980s. Since then, dronabinol (International Nonproprietary Name (INN)
for THC), has proved its clinical utility in a few countries as an adjunct therapy for that indication (Grunberg &Hesketh, 1993). Some of the early problems with unwanted side effects of oral dosages of THC have been remedied
through the availability of dronabinol capsules with half the dosage of the earlier formulation.
12.3

Stimulation of food intake in AIDS wasting syndrome

In the USA, approximately 16 per cent (about 14 000 people) of the total AIDS population suffer from theprogressive anorexia and weight loss known as AIDS wasting syndrome. Dronabinol has been approved by the US
Food and Drug Administration as a food intake stimulant for AIDS patients suffering from wasting syndrome, basedon a well-controlled double-blind, randomized clinical study with AIDS patients (Plasse et al., 1991). Anothercontrolled, double-blind, randomized trial is currently being conducted to compare the efficacy of dronabinol andmegestrol acetate, a synthetic hormone, in treating the wasting syndrome.
12.4

Other areas of therapeutic potential

While THC has long been known to reduce the increased intra ocular pressure of glaucoma, it has not been fully
studied therapeutically for this indication. This has been because of concern over the long-term
ocular and systemic effects of THC use, especially in older individuals who are the most frequentvictims of glaucoma.
Early studies had shown that cannabinoids were no more effective than other drugs used as analgesics and that reliefof pain was achieved only at doses that induced severe side effects in animals. Some newly synthesized cannabinoids
are extremely potent analgesics; however, separation of the analgesic and side effects in humans remains to be
demonstrated. Further experiments with these compounds may illuminate not only their mechanisms of action, butalso the body’s multiple mechanisms of pain reception and blockade.
Animal studies have suggested other possible therapeutic applications of THC or other cannabinoids in various
disorders. In studies with human subjects, however, cannabinoids have not yet been proven useful in the treatmentof convulsant or movement disorders or in treating multiple sclerosis or asthma. There are also reports of anantidepressant effect, and some patients may indeed use cannabis to >self-treat= depressive symptoms (Gruber et al.,1997), but these need to be better evaluated.
12.5

Therapeutic potential for cannabis

The therapeutic uses of THC described above have led to discussions about the therapeutic potential of cannabis
itself, although little research exists in this area and satisfactory clinical studies have not been conducted. In order
to explore possible therapeutic uses of cannabis, several scientific issues need to be considered, including thestandardization of cannabis preparations required for some types of clinical and pre-clinical studies, the difficulties
inherent in the study of smoking as the mode of administration of a substance, the need for a comparable placebo>cigarette= which would not be easily identified by experimental subjects and patients in controlled trials, the large
number of patients which would be needed to study the comparative efficacy of smoking cannabis compared withother cannabinoids and other therapeutic agents, and the possibility of using alternative delivery systems which could
avoid cannabis smoking and the other components of such a smokable form. In addition, the broader implicationsof such research on cannabis control policies would need to be carefully considered.

WHO/MSA/PSA/97.4
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13.

Comparing cannabis with other drugs

The group of experts who prepared the review of the current knowledge about cannabis in 1995 included a sectionin the draft report which attempted to compare what is known about the health effects of cannabis to the healthhazards of a variety of licit and illicit drugs with psychoactive effects such as alcohol, tobacco and opiates.
However, the reliability and public health significance of such comparisons are doubtful. Users of one drug are more
likely to be users of one or more other drugs, and the risks of combined use are not necessarily identical to, or thesum of, the risks associated with the use of the individual drugs. Moreover, the hazards linked to the use of any drug
are strongly influenced by such factors as the social and cultural context of drug use in the community, the politicaland economic context, availability of various psychoactive substances, preparation and dose, route of administration,
frequency of use, and associated life style. The quantitative risks of cannabis use are largely unknown in the absenceof reliable epidemiological studies, and therefore such comparisons tend to be more speculative than scientific.
In addition, since the proportion of the population that uses cannabis regularly over a period of years is currently
much smaller than the proportions that use alcohol or tobacco in a comparable way, the magnitude of the publichealth hazard based purely on such exposure considerations is likely to be lower than that posed by alcohol or
tobacco. However, it must be emphasized that most users of cannabis also use other drugs. There is no a priori
reason to reject the likelihood that the risks of multiple drug use are additive. From a public health perspective,
therefore, it may be more useful to assess the total risk resulting from all drug use, including that of cannabis.
Additionally, the public health significance of cannabis use in developing societies is even less well understood, given
the lack of research, as indeed are the hazards of alcohol and tobacco. Comparisons of the health effects ofpsychoactive substance use in these populations are likely to be of limited validity.

14.

Summary

Acute health effects of cannabis use
The acute effects of cannabis use have been recognized for many years, and recent studies have confirmed andextended earlier findings. These may be summarized as follows:-cannabis impairs cognitive development (capabilities of learning), including associative processes; free recallof previously learned items is often impaired when cannabis is used both during learning and recall periods;-

cannabis impairs psychomotor performance in a wide variety of tasks, such as motor coordination, divided
attention, and operative tasks of many types; human performance on complex machinery can be impaired
for as long as 24 hours after smoking as little as 20mg of THC in cannabis; there is an increased risk ofmotor vehicle accidents among persons who drive when intoxicated by cannabis

Chronic health effects of cannabis use
The chronic use of cannabis produces additional health hazards including:-selective impairments of cognitive functioning which include the organization and integration of complex
information involving various mechanisms of attention and memory processes;-

prolonged use may lead to greater impairment, which may not recover with cessation of use, and which could
affect daily life functions;WHO/MSA/PSA/97.4
page 30-

development of a cannabis dependence syndrome characterized by a loss of control over cannabis use islikely in chronic users;-

cannabis use can exacerbate schizophrenia in affected individuals;-

epithelial injury of the trachea and major bronchi is caused by long-term cannabis smoking;-airway injury, lung inflammation, and impaired pulmonary defence against infection from persistentcannabis consumption over prolonged periods;-

heavy cannabis consumption is associated with a higher prevalence of symptoms of chronic bronchitis anda higher incidence of acute bronchitis than in the non-smoking cohort;-cannabis use during pregnancy is associated with impairment in fetal development leading to a reduction
in birth weight.

-

cannabis use during pregnancy may lead to postnatal risk of rare forms of cancer although more researchis needed in this area.

The health consequences of cannabis use in developing countries are largely unknown because of limited and nonsystematic research, but there is no reason a priori to expect that biological effects on individuals in these
populations would be substantially different to what has been observed in developed countries. However, otherconsequences might be different given the cultural and social differences between countries.
Therapeutic uses of cannabinoids
Several studies have demonstrated the therapeutic effects of cannabinoids for nausea and vomiting in the advanced
stages of illnesses such as cancer and AIDS. Dronabinol (tetrahydrocannabinol) has been available by prescription
for more than a decade in the USA. Other therapeutic uses of cannabinoids are being demonstrated by controlled
studies, including treatment of asthma and glaucoma, as an antidepressant, appetite stimulant, anticonvulsant andantispasmodic, research in this area should continue. For example, more basic research on the central and peripheral
mechanisms of the effects of cannabinoids on gastrointestinal function may improve the ability to alleviate nausea
and emesis. More research is needed on the basic neuropharmacology of THC and other cannabinoids so that better
therapeutic agents can be found.

15.

Recommendations for future researchInformation on the effects of cannabis on physical and psychological functioning has increased greatly as has
knowledge of the extent and patterns of use. However there is still a need for further research in several important
areas including clinical and epidemiological research on human health effects, chemistry and pharmacology, andresearch into the therapeutic uses of cannabinoids. Moreover, there are important gaps in knowledge about thehealth consequences of cannabis use. The most pressing issues for further research are summarized below.
15.1

Clinical and epidemiological research

There is a need for more data on the patterns of cannabis use and resulting problems particularly in developing
countries. Such research would benefit from greater use of simplified and comparable methods of gathering datain these countries, so that information collected in different countries can be compared. Few countries have cohort
studies of cannabis use patterns which are important in order to assess the natural history of cannabis use and theWHO/MSA/PSA/97.4
page 31

reasons for starting and stopping cannabis use at all levels of consumption. There is also a need for case-control
studies comparing those experiencing cannabis problems, with people who have, and do not have, alcohol and otherpsychoactive substance use problems.
There is a need for controlled studies investigating the relationships between cannabis use, schizophrenia and otherserious mental disorders. In particular, there is a need for intervention studies of schizophrenic persons to see
whether stopping cannabis use improves their outcomes in treatment.
Insufficient research has been undertaken on the >amotivational= syndrome which may or may not result from heavy
cannabis use. It is not clear that such a syndrome exists, even though heavy cannabis use is sometimes associatedwith reduced motivation to succeed in school and at work. Also, new research is needed to show whether the reduced
motivation seen in some cannabis users is due to other psychoactive substance use and whether it precedes cannabis
use. How reduced motivation relates to psychological problems is still unknown and requires further research.
Research on chronic and residual cannabis effects is also needed. The pharmacokinetics of chronic cannabis use
in humans are poorly described and this lack of knowledge restricts the ability of researchers to relate drugconcentrations in blood or other fluids and observed effects.
The prevalence and consequences of dependence on cannabis is a major area requiring further research. There is aneed for better delineation of the clinical features of cannabis dependence and for studies of its responsiveness tointerventions aimed at assisting users to stop.
Further studies are required on fertility effects in cannabis users, in view of the high rate of use during the early
reproductive years. Researchers investigating infertility should be encouraged to study the effects of cannabis use.
In addition, given the importance of the issue of in utero exposure, more research on fetal metabolism, especiallyof premature infants, is needed. Another priority would be replication of the case-control studies of maternalcannabis use and childhood cancers.
Further clinical and epidemiological research is required on the effects of cannabis on respiratory function andrespiratory diseases. More studies are needed to show whether cannabis affects the risk of lung malignancies andat what level of use that may occur. In addition, more studies are needed to clarify the rather different results of
pulmonary histopathological studies in animals and man.
More clinical and experimental research is needed on the effects of cannabis on the immunological function. Futurestudies should be aimed at establishing the relationship between cannabinoid-induced immunomodulation and altered
host resistance to microbes and tumours, and exploring the role of cannabinoid receptors on host immunity and in
the regulation of the normal immune response. More clarity should also be sought concerning the molecularmechanisms responsible for immune effects, including both cannabinoid receptor and non-receptor events.
The possibility that chronic cannabis use has adverse effects on the cardiovascular system should have a priority inepidemiological research. There is evidence from laboratory studies that cannabinoids have pronounced acute effects
on cardiovascular functioning and it is known that cannabis smoke is qualitatively similar to tobacco smoke which
is a known serious hazard for cardiovascular diseases. The fact that the cohort of cannabis users who initiated use
in the early 1970s are now entering the period of maximum risk for cardiovascular disease suggests that it would
be timely to conduct case-control studies of cardiovascular disease and cannabis use.
Frequently the risks of cannabis use are compared with those of other drugs such as alcohol and tobacco. However,the science to evaluate health risks of cannabis use is much less extensive and much less conclusive than thevoluminous research that has been conducted on these psychoactive substances. In addition, there is insufficientresearch on cannabis-related mortality from accidents and other causes to permit proper comparisons. Morecomparative epidemiological studies of cannabis and other drugs are needed to assess their relative mortality andmorbidity risks at different levels of use.

WHO/MSA/PSA/97.4
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15.2

Chemistry, pharmacology and physiology

Certain approaches to the planning and design of research would improve an understanding of cannabis effects ina number of areas. For example pharmacological research is needed to understand to what extent the varieties ofcannabis preparations affect humans and animals differently. Attention needs to be given to elucidating doseresponse relationships rather than simply examining the effects of a single dose. More information is needed on how
dose-response relationships for most effects vary for different species. Further research is needed on what animaldose for a given type of toxicity corresponds to what human dose in order to ensure comparability in the two typesof studies.
Several important research issues remain ambiguous with regard to the effects of cannabis on human physiology.
For example the sites of action for appetite stimulants and the antiemetic effects of cannabis are unknown. Anotherhigh priority is to identify the physiological roles of the endogenous cannabinoid system. Also, some future researchshould be directed towards establishing whether a specific interaction occurs between the endogenous cannabinoid
and glucocorticoids. This will require multifaceted efforts by chemists, neurochemists and molecular biologists.
Strategies include the identification of additional endogenous cannabinoids, syntheses of analogues with selectedpharmacological profiles and development of genetically-altered experimental animals which are devoid ofcannabinoid receptors. It is imperative to determine whether the endogenous cannabinoid system meets all therequirements for a neurotransmittal system.
Some effort has been made to determine the relationship between THC concentrations in blood and other fluids andthe degree of behavioural impairment produced. Although this task is difficult, efforts should continue with the goalof defining a concentration-effect relationship as clearly as has been done for alcohol.
Further development of cognitive and psychomotor tests for controlled studies that are sensitive to the performance
effects of cannabis use and that reflect the complexity of specific daily functions (i.e. driving, learning, reasoning)
also need additional research.

WHO/MSA/PSA/97.4
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Annex 1
EXPERT WORKING GROUP ON
HEALTH EFFECTS OF CANNABIS USE
Geneva, 22-24 May 1995

List of Participants

Dr Robert Ali, Treatment Services, Drug and Alcohol Services Council, Adelaide, Australia
Dr S. M. Channabasavanna, National Institute of Mental Health and Neurosciences, Bangalore, India
Dr William Corrigall, Addiction Research Foundation, Toronto, Canada
Dr Wayne Hall, National Drug and Alcohol Research Center, University of New South Wales, Kensington, Australia
Dr Christine R Hartel, National Institute on Drug Abuse (NIDA), Washington D.C., USA
Dr Harold Kalant, Addiction Research Foundation, Toronto, Canada (Chairman)

Dr Billy R Martin, College on Problems of Drug Dependence, Medical College of Virginia, Richmond, USA
Dr Mehdi Paes, Ar-Razi Hospital, Sale, Morocco
Dr Reginald Smart, Addiction Research Foundation, Ontario, Canada

Representatives of other organizations
Dr Kalman Szendrei, United Nationals Drug Control Programme (UNDCP), Vienna, Austria

Secretariat
Dr Mario Argandoña, Treatment and Care Unit, PSA
Dr Andrew Ball, Treatment and Care, PSA
Dr Pia Bergendahl, PSA
Mr Hans Emblad, PSA
Mr Tokuo Yoshida, Regulatory and Control Unit, PSA

WHO/MSA/PSA/97.4
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Annex 2
EXPERT WORKING GROUP ON
HEALTH EFFECTS OF CANNABIS USE
Geneva, 22-24 May 1995

Background papers
Beardsley PM, Kelly TH Acute effects of cannabis on human behaviour and CNS function: update ofexperimental studies
Channabasavana SM, Paes M, Hall W Mental and behavioural disorders due to cannabis use
Chesher G, Hall W The effects of cannabis on the cardiovascular and gastrointestinal systems
Hall, W Assessing the health and psychological effects of cannabis use
Hall W, Johnston L Donnelly N
consequences

Epidemiological evidence on patterns of cannabis use and their healthHall W, Room R A comparative appraisal of the health and psychological consequences of alcohol, cannabis,
nicotine and opiate use
Hartel CR Medical uses of marijuana
Hutchings DE, Fried PA Cannabis during pregnancy: neurobehavioural effects in animals and humans
Klein TW Cannabis and Immunity
MacPhee DG Effects of marijuana on cell nuclei: a review of the literature
Martin BR, Cone EJ Chemistry and Pharmacology
Murphy LL Cannabis effects on endocrine and reproductive function
Smiley A Marijuana: on road and driving simulator studies
Solowij N The long term effects of cannabis on the central nervous system I. Brain function and neurotoxicity
Solowij N The long term effects of cannabis on the central nervous system II. Cognitive functioning
Tashkin DP Cannabis effects on the respiratory system: review of the literature*****