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Marijuana is the most commonly used

illicit drug in the United States. A dry,

shredded green/brown mix of flowers,

stems, seeds, and leaves of the hemp

plant Cannabis sativa, it usually is smoked

as a cigarette (joint, nail), or in a pipe

(bong). It also is smoked in blunts, which

are cigars that have been emptied of

tobacco and refilled with marijuana, often

in combination with another drug. Use

also might include mixing marijuana in

food or brewing it as a tea. As a more

concentrated, resinous form it is called

hashish and, as a sticky black liquid,

hash oil. Marijuana smoke has a pungent

and distinctive, usually sweet-and-sour

odor. There are countless street terms for

marijuana including pot, herb, weed,

grass, widow, ganja, and hash, as well as

terms derived from trademarked varieties

of cannabis, such as Bubble Gum®

,

Northern Lights®

, Juicy Fruit®, Afghani #1®,

and a number of Skunk varieties. 

The main active chemical in marijuana is

THC (delta-9-tetrahydrocannabinol). The

membranes of certain nerve cells in the

brain contain protein receptors that bind

to THC. Once securely in place, THC 

kicks off a series of cellular reactions that

ultimately lead to the high that users

experience when they smoke marijuana. 

Extent of Use ———

There were an estimated 2.6 million new

marijuana users in 2001. This number is

similar to the numbers of new users each

year since 1995, but above the number in

1990 (1.6 million). In 2002, over 14 million Americans age 12 and older used

marijuana at least once in the month prior

to being surveyed, and 12.2 percent of

past year marijuana users used marijuana

on 300 or more days in the past 12

months. This translates into 3.1 million

people using marijuana on a daily or

almost daily basis over a 12-month

period(1). 

The percentage of youth age 12 to 17 who

had ever used marijuana declined slightly

from 2001 to 2002 (21.9 to 20.6 percent).

Among adults age 18 to 25, the rate increased slightly from 53.0 percent to 53.8

percent in 2002. The percentage of young

adults age 18 to 25 who had ever used

marijuana was 5.1 percent in 1965, but

increased steadily to 54.4 percent in 1982.

Although the rate for young adults declined

somewhat from 1982 to 1993, it did not

drop below 43 percent and actually

increased to 53.8 percent by 2002(1).

Forty-two percent of youth age 12 or 13

and 24.1 percent age 16 or 17 perceived

Marijuana
smoking marijuana once a month as a

great risk. Slightly more than half of youth

age 12 to 17 indicated that it would be

fairly or very easy to obtain marijuana,

but only 26.0 percent of 12- or 13-yearolds indicated the same thing. However,

79.0 percent of those age 16 or 17 

indicated that it would be fairly or very

easy to obtain marijuana(1).

Prevalence of lifetime, past year, and past

month marijuana use declined among 

students in 8th, 10th, and 12th grades in

2003. However, the declines in 12-month

prevalence reached statistical significance

only in 8th-graders; past year use has

declined by nearly one-third since 1996(2).

All three grades showed an increase in

perceived risk for regular marijuana use.

This finding represents a welcome turnaround in this perception, which has been

in decline in all grades over the past 1 or

2 years(3). 

In 2002, marijuana was the third most

commonly abused drug mentioned in

drug-related hospital emergency department (ED) visits in the continental United

States. Marijuana mentions rose significantly (24%) from 2000 to 2002, but

showed no significant increase since

2001. Taking changes in population into

account, marijuana mentions increased

139 percent from 1995 to 2002(4).

Effects on the Brain

Scientists have learned a great deal about

how THC acts in the brain to produce its

many effects. When someone smokes

marijuana, THC rapidly passes from the

lungs into the bloodstream, which carries

the chemical to organs throughout the

body, including the brain.

In the brain, THC connects to specific sites

called cannabinoid receptors on nerve

cells and influences the activity of those

cells. Some brain areas have many

cannabinoid receptors; others have few 

or none. Many cannabinoid receptors are

found in the parts of the brain that influence

pleasure, memory, thought, concentration,

sensory and time perception, and coordinated movement(5).

The short-term effects of marijuana can

include problems with memory and 

learning; distorted perception; difficulty 

in thinking and problem solving; loss of

coordination; and increased heart rate.

Research findings for long-term marijuana

use indicate some changes in the brain

similar to those seen after long-term use

of other major drugs of abuse. For example, cannabinoid (THC or synthetic forms

of THC) withdrawal in chronically

exposed animals leads to an increase in

the activation of the stress-response 

system(6) and changes in the activity of

nerve cells containing dopamine(7).

Dopamine neurons are involved in the

regulation of motivation and reward, and

are directly or indirectly affected by all

drugs of abuse.

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Effects on the Heart

One study has indicated that a user’s risk

of heart attack more than quadruples in

the first hour after smoking marijuana(8).

The researchers suggest that such an

effect might occur from marijuana’s effects

on blood pressure and heart rate and

reduced oxygen-carrying capacity of

blood.

Effects on the Lungs

A study of 450 individuals found that

people who smoke marijuana frequently

but do not smoke tobacco have more

health problems and miss more days of

work than nonsmokers(9). Many of the

extra sick days among the marijuana

smokers in the study were for respiratory

illnesses.

Even infrequent use can cause burning and

stinging of the mouth and throat, often

accompanied by a heavy cough. Someone

who smokes marijuana regularly may

have many of the same respiratory problems

that tobacco smokers do, such as daily

cough and phlegm production, more 

frequent acute chest illness, a heightened

risk of lung infections, and a greater tendency to obstructed airways(10). Smoking

marijuana increases the likelihood of

developing cancer of the head or neck,

and the more marijuana smoked the

greater the increase(11). A study comparing

173 cancer patients and 176 healthy

individuals produced strong evidence that

marijuana smoking doubled or tripled the

risk of these cancers.

Marijuana use also has the potential to

promote cancer of the lungs and other

parts of the respiratory tract because it

contains irritants and carcinogens(12, 13). In

fact, marijuana smoke contains 50 to 70

percent more carcinogenic hydrocarbons

than does tobacco smoke(14). It also produces

high levels of an enzyme that converts

certain hydrocarbons into their carcinogenic

form—levels that may accelerate the

changes that ultimately produce malignant

cells(15). Marijuana users usually inhale

more deeply and hold their breath longer

than tobacco smokers do, which increases

the lungs’ exposure to carcinogenic

smoke. These facts suggest that, puff for

puff, smoking marijuana may increase the

risk of cancer more than smoking tobacco.

Other Health Effects

Some of marijuana’s adverse health

effects may occur because THC impairs

the immune system’s ability to fight off

infectious diseases and cancer. In laboratory experiments that exposed animal and

human cells to THC or other marijuana

ingredients, the normal disease-preventing

reactions of many of the key types of

immune cells were inhibited(16). In other

studies, mice exposed to THC or related

substances were more likely than unexposed mice to develop bacterial infections

and tumors(17, 18).

Effects of Heavy Marijuana Use

on Learning and Social Behavior

Depression(19), anxiety(20), and personality

disturbances(21) have been associated with
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marijuana use. Research clearly demonstrates that marijuana has potential to

cause problems in daily life or make a

person’s existing problems worse. Because

marijuana compromises the ability to

learn and remember information, the

more a person uses marijuana the more

he or she is likely to fall behind in accumulating intellectual, job, or social skills.

Moreover, research has shown that marijuana’s adverse impact on memory and

learning can last for days or weeks after

the acute effects of the drug wear off(22, 23).

Students who smoke marijuana get lower

grades and are less likely to graduate

from high school, compared with their

non-smoking peers(24, 25, 26, 27). A study of

129 college students found that, for heavy

users of marijuana (those who smoked the

drug at least 27 of the preceding 30

days), critical skills related to attention,

memory, and learning were significantly

impaired even after they had not used the

drug for at least 24 hours(28). The heavy

marijuana users in the study had more

trouble sustaining and shifting their attention and in registering, organizing, and

using information than did the study participants who had used marijuana no

more than 3 of the previous 30 days. As

a result, someone who smokes marijuana

every day may be functioning at a

reduced intellectual level all of the time.

More recently, the same researchers

showed that the ability of a group of

long-term heavy marijuana users to recall

words from a list remained impaired for a

week after quitting, but returned to normal within 4 weeks(29). Thus, it is possible

that some cognitive abilities may be

restored in individuals who quit smoking

marijuana, even after long-term heavy use.

Workers who smoke marijuana are more

likely than their coworkers to have problems on the job. Several studies associate

workers’ marijuana smoking with

increased absences, tardiness, accidents,

workers’ compensation claims, and job

turnover. A study of municipal workers

found that those who used marijuana on

or off the job reported more “withdrawal

behaviors”—such as leaving work without

permission, daydreaming, spending work

time on personal matters, and shirking

tasks—that adversely affect productivity

and morale(30). In another study, marijuana users reported that use of the drug

impaired several important measures of

life achievement including cognitive abilities, career status, social life, and physical

and mental health(31).

Effects on Pregnancy

Research has shown that babies born to

women who used marijuana during their

pregnancies display altered responses to

visual stimuli, increased tremulousness,

and a high-pitched cry, which may indicate neurological problems in development(32). During infancy and preschool

years, marijuana-exposed children have

been observed to have more behavioral

problems than unexposed children and
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poorer performance on tasks of visual perception, language comprehension, sustained

attention, and memory(33, 34). In school,

these children are more likely to exhibit

deficits in decision-making skills, memory,

and the ability to remain attentive(35, 36, 37).

Addictive Potential

Long-term marijuana use can lead to

addiction for some people; that is, they

use the drug compulsively even though it

interferes with family, school, work, and

recreational activities. Drug craving and

withdrawal symptoms can make it hard

for long-term marijuana smokers to stop

using the drug. People trying to quit

report irritability, sleeplessness, and anxiety(38). They also display increased aggression on psychological tests, peaking

approximately one week after the last use

of the drug(39).

Genetic Vulnerability

Scientists have found that whether an

individual has positive or negative sensations after smoking marijuana can be

influenced by heredity. A 1997 study

demonstrated that identical male twins

were more likely than non-identical male

twins to report similar responses to marijuana use, indicating a genetic basis for

their response to the drug(40). (Identical

twins share all of their genes.)

It also was discovered that the twins’

shared or family environment before age

18 had no detectable influence on their

response to marijuana. Certain environmental factors, however, such as the

availability of marijuana, expectations

about how the drug would affect them,

the influence of friends and social 

contacts, and other factors that differentiate

experiences of identical twins were found

to have an important effect. 

Treating Marijuana Problems

The latest treatment data indicate that, in

2000, marijuana was the primary drug 

of abuse in about 15 percent (236,638) of

all admissions to treatment facilities in the

United States. Marijuana admissions were

primarily male (76 percent), White (57

percent), and young (46 percent under 20

years old). Those in treatment for primary

marijuana use had begun use at an early

age; 56 percent had used it by age 14

and 92 percent had used it by 18(41).

One study of adult marijuana users found

comparable benefits from a 14-session

cognitive-behavioral group treatment and

a 2-session individual treatment that

included motivational interviewing and

advice on ways to reduce marijuana use.

Participants were mostly men in their

early thirties who had smoked marijuana

daily for more than 10 years. By increasing

patients’ awareness of what triggers their

marijuana use, both treatments sought to

help patients devise avoidance strategies.

Use, dependence symptoms, and psychosocial problems decreased for at least 1

year following both treatments; about 30

percent of users were abstinent during the

last 3-month followup period(42).
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Another study suggests that giving

patients vouchers that they can redeem

for goods—such as movie passes, sporting

equipment, or vocational training—may

further improve outcomes(43).

Although no medications are currently

available for treating marijuana abuse,

recent discoveries about the workings of

the THC receptors have raised the possibility

of eventually developing a medication

that will block the intoxicating effects of

THC. Such a medication might be used to

prevent relapse to marijuana abuse by

lessening or eliminating its appeal.

Percentage of 8th-Graders Who Have Used Marijuana:

Monitoring the Future Study, 2003

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Ever 

Used 16.7% 19.9% 23.1% 22.6% 22.2% 22.0% 20.3% 20.4% 19.2% 17.5%

Used 

in Past 

Year 13.0 15.8 18.3 17.7 16.9 16.5 15.6 15.4 14.6 12.8

Used 

in Past 

Month 7.8 9.1 11.3 10.2 9.7 9.7 9.1 9.2 8.3 7.5

Daily 

Use in 

Past 

Month 0.7 0.8 1.5 1.1 1.1 1.4 1.3 1.3 1.2 1.0

Percentage of 10th-Graders Who Have Used Marijuana:

Monitoring the Future Study, 2003

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Ever 

Used 30.4% 34.1% 39.8% 42.3% 39.6% 40.9% 40.3% 40.1% 38.7% 36.4%

Used 

in Past 

Year 25.2 28.7 33.6 34.8 31.1 32.1 32.2 32.7 30.3 28.2
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1979 1991 1992 1993 1994 1995 1996

Ever Used 60.4% 36.7% 32.6% 35.3% 38.2% 41.7% 44.9%

Used in 

Past Year 50.8 23.9 21.9 26.0 30.7 34.7 35.8

Used in 

Past Month 36.5 13.8 11.9 15.5 19.0 21.2 21.9

Daily Use in

Past Month 10.3 2.0 1.9 2.4 3.6 4.6 4.9

1997 1998 1999 2000 2001 2002 2003

Ever Used 49.6% 49.1% 49.7% 48.8% 49.0% 47.8% 46.1%

Used in 

Past Year 38.5 37.5 37.8 36.5 37.0 36.2 34.9

Used in 

Past Month 23.7 22.8 23.1 21.6 22.4 21.5 21.2

Daily Use in 

Past Month 5.8 5.6 6.0 6.0 5.8 6.0 6.0

Percentage of 12th-Graders Who Have Used Marijuana

Monitoring the Future Study, 2003

Cont.

These data are from the 2003 Monitoring the Future (MTF) Survey, funded by National Institute on Drug Abuse, National

Institutes of Health, DHHS, and conducted by the University of Michigan’s Institute for Social Research. The survey has

tracked 12th-graders’ illicit drug use and related attitudes since 1975; in 1991, 8th- and 10th-graders were added to the

study. The latest data are online at www.drugabuse.gov.

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Used 

in Past 

Month 15.8 17.2 20.4 20.5 18.7 19.4 19.7 19.8 17.8 17.0

Daily 

Use in 

Past 

Month 2.2 2.8 3.5 3.7 3.6 3.8 3.8 4.5 3.9 3.6
March 2004 Page 8 of 10

1 NSDUH (formerly known as the National Household Survey on Drug Abuse) is an annual survey conducted by the

Substance Abuse and Mental Health Services Administration. Copies of the latest survey are available from the National

Clearinghouse for Alcohol and Drug Information at 1-800-729-6686.

2 These data are from the 2003 Monitoring the Future Survey, funded by the National Institute on Drug Abuse, National

Institutes of Health, DHHS, and conducted by the University of Michigan’s Institute for Social Research. The survey has

tracked 12th-graders’ illicit drug use and related attitudes since 1975; in 1991, 8th- and 10th-graders were added to the

study. The latest data are online at www.drugabuse.gov. 

3 These data are from the 2003 Monitoring the Future Survey.

4 These data are from the annual Drug Abuse Warning Network, funded by the Substance Abuse and Mental Health

Services Administration, DHHS. The survey provides information about emergency department visits that are induced by

or related to the use of an illicit drug or the nonmedical use of a legal drug. The latest data (2002) are available at 1-

800-729-6686 or online at www.samhsa.gov. 

5 Herkenham M, Lynn A, Little MD, Johnson MR, et al: Cannabinoid receptor localization in the brain. Proc Natl Acad Sci,

USA 87:1932-1936, 1990.

6 Rodriguez de Fonseca F, et al: Activation of cortocotropin-releasing factor in the limbic system during cannabinoid withdrawal. Science 276(5321):2050-2064, 1997.

7 Diana M, Melis M, Muntoni AL, et al: Mesolimbic dopaminergic decline after cannabinoid withdrawal. Proc Natl Acad

Sci 95:10269-10273, 1998.

8 Mittleman MA, Lewis RA, Maclure M, et al: Triggering myocardial infarction by marijuana. Circulation 103:2805-2809,

2001. 

9 Polen MR, Sidney S, Tekawa IS, et al: Health care use by frequent marijuana smokers who do not smoke tobacco. West

J Med 158:596-601, 1993.

10 Tashkin DP: Pulmonary complications of smoked substance abuse. West J Med 152:525-530, 1990.

11 Zhang ZF, Morgenstern H, Spitz MR, et al: Marijuana use and increased risk of squamous cell carcinoma of the head

and neck. Cancer Epidemiology, Biomarkers & Prevention 6:1071-1078, 1999.

12 Ibid ref 10.

13 Sridhar KS, Raub WA, Weatherby, NL Jr, et al: Possible role of marijuana smoking as a carcinogen in the development

of lung cancer at a young age. Journal of Psychoactive Drugs 26(3):285-288, 1994. 

14 Hoffman D, Brunnemann KD, Gori GB, et al: On the carcinogenicity of marijuana smoke. In: VC Runeckles, ed, Recent

Advances in Phytochemistry. New York. Plenum, 1975.

15 Cohen S: Adverse effects of marijuana: selected issues. Annals of the New York Academy of Sciences 362:119-124,

1981. 

16 Adams IB, Martin BR: Cannabis: pharmacology and toxicology in animals and humans. Addiction 91:1585-1614, 1996.

17 Klein TW, Newton C, Friedman H: Resistance to Legionella pneumophila suppressed by the marijuana component,

tetrahydrocannabinol. J Infectious Disease 169:1177-1179, 1994.

18 Zhu L, Stolina M, Sharma S, et al: Delta-9 tetrahydrocannabinol inhibits antitumor immunity by a CB2 receptor-mediated, cytokine-dependent pathway. J Immunology, 2000, pp. 373-380.

19 Brook JS, et al: The effect of early marijuana use on later anxiety and depressive symptoms. NYS Psychologist, January

2001, pp. 35-39.
March 2004 Page 9 of 10

20 Green BE, Ritter C: Marijuana use and depression. J Health Soc Behav 41(1):40-49, 2000.

21 Brook JS, Cohen P, Brook DW: Longitudinal study of co-occurring psychiatric disorders and substance use. J Acad Child

and Adolescent Psych 37:322-330, 1998.

22 Pope HG, Yurgelun-Todd D: The residual cognitive effects of heavy marijuana use in college students. JAMA

272(7):521-527, 1996. 

23 Block RI, Ghoneim MM: Effects of chronic marijuana use on human cognition. Psychopharmacology 100(1-2):219-228,

1993. 

24 Lynskey M, Hall W: The effects of adolescent cannabis use on educational attainment: a review. Addiction 95(11):1621-

1630, 2000.

25 Kandel DB, Davies M: High school students who use crack and other drugs. Arch Gen Psychiatry 53(1):71-80, 1996.

26 Rob M, Reynolds I, Finlayson PF: Adolescent marijuana use: risk factors and implications. Aust NZ J Psychiatry

24(1):45-56, 1990.

27 Brook JS, Balka EB, Whiteman M: The risks for late adolescence of early adolescent marijuana use. Am J Public Health

89(10):1549-1554, 1999.

28 Ibid ref 22.

29 Pope, Gruber, Hudson, et al: Neuropsychological performance in long-term cannabis users. Archives of General

Psychiatry. 

30 Lehman WE, Simpson DD: Employee substance abuse and on-the-job behaviors. Journal of Applied Psychology

77(3):309-321, 1992.

31 Gruber, AJ, Pope HG, Hudson HI, Yurgelun-Todd D: Attributes of long-term heavy cannabis users: A case control study.

Psychological Medicine 33:1415-1422, 2003.

32 Lester, BM; Dreher, M: Effects of marijuana use during pregnancy on newborn cry. Child Development 60:764-771,

1989. 

33 Fried, PA: The Ottawa prenatal prospective study (OPPS): methodological issues and findings—it’s easy to throw the

baby out with the bath water. Life Sciences 56:2159-2168, 1995.

34 Fried, PA: Prenatal exposure to marihuana and tobacco during infancy, early and middle childhood: effects and an

attempt at synthesis. Arch Toxicol Supp 17:233-60, 1995.

35 Ibid ref 33. 

36 Ibid ref 34.

37 Cornelius MD, Taylor PM, Geva D, et al: Prenatal tobacco and marijuana use among adolescents: effects on offspring

gestational age, growth, and morphology. Pediatrics 95:738-743, 1995. 

38 Kouri EM, Pope HG, Lukas SE: Changes in aggressive behavior during withdrawal from long-term marijuana use.

Psychopharmacology 143:302-308, 1999.

39 Haney M, Ward AS, Comer SD, et al: Abstinence symptoms following smoked marijuana in humans.

Psychopharmacology 141:395-404, 1999. 

40 Lyons MJ, et al: Addiction 92(4):409-417, 1997.
March 2004 Page 10 of 10

National Institutes of Health – U.S. Department of Health and Human Services

This material may be used or reproduced without permission from NIDA. Citation of the source is appreciated.

41 These data from the Treatment Episode Data Set (TEDS) 1992-2000: National Admissions to Substance Abuse

Treatment Services, November 2001, funded by the Substance Abuse and Mental Health Service Administration, DHHS.

The latest data are available at 1-800-729-6686 or online at www.samhsa.gov.

42 Stephens RS, Roffman RA, Curtin L: Comparison of extended versus brief treatments for marijuana use. J Consult 

Clin Psychol 68(5):898-908, 2000.

43 Budney AJ, Higgins ST, Radonovich KJ, et al: Adding voucher-based incentives to coping skills and motivational enhancement improves outcomes during treatment for marijuana dependence. J Consult Clin Psychol 68(6):1051-1061, 2000.