What Is 

Comorbidity?

from the director:

Comorbidity is a topic that our 

stakeholders––patients, family members, 

health care professionals, and others––

frequently ask about. It is also a topic about 

which we have insufficient information, 

so it remains a research priority for 

NIDA. This Research Report provides 

information on the state of the science in 

this area. Although a variety of diseases 

commonly co-occur with drug abuse and 

addiction (e.g., HIV, hepatitis C, cancer, 

cardiovascular disease), this report focuses 

only on the comorbidity of drug use 

disorders and other mental illnesses.* 

To help explain this comorbidity, we need 

to first recognize that drug addiction is a 

mental illness. It is a complex brain disease 

characterized by compulsive, at times 

uncontrollable drug craving, seeking, and 

use despite devastating consequences—

behaviors that stem from drug-induced 

changes in brain structure and function. 

These changes occur in some of the same 

brain areas that are disrupted in other 

mental disorders, such as depression, 

anxiety, or schizophrenia. It is therefore not 

surprising that population surveys show a 

high rate of co-occurrence, or comorbidity, 

between drug addiction and other mental 

illnesses. While we cannot always prove a 

connection or causality, we do know that 

certain mental disorders are established 

risk factors for subsequent drug abuse—

and vice versa. 

It is often difficult to disentangle the 

overlapping symptoms of drug addiction 

and other mental illnesses, making 

diagnosis and treatment complex. Correct 

diagnosis is critical to ensuring appropriate 

and effective treatment. Ignorance of or 

failure to treat a comorbid disorder can 

jeopardize a patient’s chance of recovery. 

We hope that our enhanced understanding 

of the common genetic, environmental, and 

neural bases of these disorders—and the 

dissemination of this information—will lead 

to improved treatments for comorbidity and 

will diminish the social stigma that makes 

patients reluctant to seek the treatment 

they need. 

Nora D. Volkow, M.D.

Director

National Institute on Drug Abuse

Comorbidity: 

Addiction and Other Mental Illnesses

Is there a relationship 

between childhood 

ADHD and later drug 

abuse? See page 2.

When two disorders or illnesses occur in the same 

person, simultaneously or sequentially, they 

are described as comorbid. Comorbidity also 

implies interactions between the illnesses that affect the 

course and prognosis of both. 

*Since the focus of this report is on comorbid drug use disorders and 

other mental illnesses, the terms “mental illness” and “mental disorders” 

will refer here to disorders other than substance use disorders, such as 

depression, schizophrenia, anxiety, and mania. The terms “dual diagnosis,” 

“mentally ill chemical abuser,” and “co-occurrence” are also used to refer to 

drug use disorders that are comorbid with other mental illnesses.

continued inside
Childhood ADHD 

and Later Drug 

Problems 

Numerous studies have 

documented an increased risk for 

drug use disorders in youth with 

untreated ADHD, although some 

suggest that only a subset of 

these individuals are vulnerable: 

those with comorbid conduct 

disorders. Given this linkage, it is 

important to determine whether 

effective treatment of ADHD 

could prevent subsequent drug 

abuse and associated behavioral 

problems. Treatment of childhood 

ADHD with stimulant medications 

such as methylphenidate or 

amphetamine reduces the 

impulsive behavior, fidgeting, 

and inability to concentrate 

that characterize ADHD. Yet, 

some physicians and parents 

have expressed concern that 

treating childhood ADHD with 

stimulants might increase a child’s 

vulnerability to drug abuse later in 

life. Recent reviews of long-term 

studies of children with ADHD 

who were treated with stimulant 

medications (e.g., Adderal, Ritalin, 

Concerta) found no evidence for 

this increase. However, most of 

these studies have methodological 

limitations, including small sample 

sizes and nonrandomized study 

designs, indicating that more 

research is needed, particularly 

with adolescents.

Research Report Series Comorbidity

Is Drug Addiction 

a Mental Illness?

Yes, because addiction changes 

the brain in fundamental ways, 

disturbing a person’s normal 

hierarchy of needs and desires 

and substituting new priorities 

connected with procuring and 

using the drug. The resulting 

compulsive behaviors that 

override the ability to control 

impulses despite the consequences 

are similar to hallmarks of other 

mental illnesses. 

In fact, the DSM, which is the 

definitive resource of diagnostic 

criteria for all mental disorders, 

includes criteria for drug use 

disorders, distinguishing between 

two types: drug abuse and drug 

dependence. Drug dependence

is synonymous with addiction. 

By comparison, the criteria for 

drug abuse hinge on the harmful 

consequences of repeated use but 

do not include the compulsive 

use, tolerance (i.e., needing higher 

doses to achieve the same effect), 

or withdrawal (i.e., symptoms that 

occur when use is stopped) that 

can be signs of addiction.

How Common 

Are Comorbid 

Drug Use and 

Other Mental 

Disorders?

Many people who regularly 

abuse drugs are also diagnosed 

with mental disorders and vice 

versa. The high prevalence of this 

comorbidity has been documented 

in multiple national population 

surveys since the 1980s. Data 

show that persons diagnosed 

with mood or anxiety disorders 

are about twice as likely to suffer 

also from a drug use disorder 

(abuse or dependence) compared 

with respondents in general. The 

same is true for those diagnosed 

with an antisocial syndrome, 

such as antisocial personality 

or conduct disorder. Similarly, 

persons diagnosed with drug 

disorders are roughly twice as 

likely to suffer also from mood 

and anxiety disorders (see page 3, 

“Overlapping Conditions— Shared 

Vulnerability”). 

Gender is also a factor in the 

specific patterns of observed 

comorbidities. For example, 

the overall rates of abuse and 

dependence for most drugs tend 

to be higher among males than 

females. Further, males are more 

likely to suffer from antisocial 

personality disorder, while women 

have higher rates of mood and 

anxiety disorders, all of which are 

risk factors for substance abuse. 

Addiction 

changes the 

brain, disturbing 

the normal 

hierarchy of 

needs and 

desires.

2 NIDA Research Report Series
Why Do Drug Use 

Disorders Often CoOccur With Other 

Mental Illnesses?

The high prevalence of comorbidity 

between drug use disorders and other 

mental illnesses does not mean that one 

caused the other, even if one appeared 

first. In fact, establishing causality or 

directionality is difficult for several 

reasons. Diagnosis of a mental disorder 

may not occur until symptoms have 

progressed to a specified level (per 

DSM); however, subclinical symptoms 

may also prompt drug use, and 

imperfect recollections of when drug use 

or abuse started can create confusion as 

to which came first. Still, three scenarios 

deserve consideration:

1. Drugs of abuse can cause abusers to 

experience one or more symptoms of 

another mental illness. The increased 

risk of psychosis in some marijuana 

abusers has been offered as evidence 

for this possibility. 

2. Mental illnesses can lead to drug 

abuse. Individuals with overt, mild, 

or even subclinical mental disorders 

may abuse drugs as a form of selfmedication. For example, the use 

of tobacco products by patients 

with schizophrenia is believed to 

lessen the symptoms of the disease 

and improve cognition (see page 

4, “Smoking and Schizophrenia: 

Self-Medication or Shared Brain 

Circuitry?”).

3. Both drug use disorders and 

other mental illnesses are caused 

by overlapping factors such as 

underlying brain deficits, genetic 

vulnerabilities, and/or early exposure 

to stress or trauma. 

All three scenarios probably contribute, 

in varying degrees, to how and whether 

specific comorbidities manifest 

themselves.

0%

5

10

15

20

25

Any Drug Opioids Amphetamines Cocaine Marijuana

Any mood disorder

Any anxiety disorder

All respondents

High Prevalence of Drug Abuse and Dependence 

Among Individuals With Mood and Anxiety Disorders

0%

10

20

30

40

50

Any drug use disorder

All respondents

Mood Disorders Anxiety Disorders

Because mood disorders 

increase vulnerability to 

drug abuse and addiction, 

the diagnosis and 

treatment of the mood 

disorder can reduce the 

risk of subsequent drug 

use. Because the inverse 

may also be true, the 

diagnosis and treatment 

of drug use disorders 

may reduce the risk of 

developing other mental 

illnesses and, if they do 

occur, lessen their severity 

or make them more 

amenable to effective 

treatment. Finally, 

because more than 40 

percent of the cigarettes 

smoked in this country 

are smoked by individuals 

with a psychiatric disorder, 

such as major depressive 

disorder, alcoholism, posttraumatic stress disorder 

(PTSD), schizophrenia, or 

bipolar disorder, smoking 

by patients with mental 

illness contributes 

greatly to their increased 

morbidity and mortality.

Higher Prevalence of Mental Disorders 

Among Patients With Drug Use Disorders

Overlapping Conditions—Shared Vulnerability

0%

10

20

30

40

50

60

70

80

Drug 

Abuse or 

Dependence

Bipolar 

Disorder

Post-Traumatic 

Stress 

Disorder

Alcohol 

Abuse or 

Dependence

Major

 Depression

No Mental 

Illness

Current smokers

Data in top two graphs reprinted from the National Epidemiologic Survey on Alcohol and Related Conditions (Conway et al., 2006). 

Data in bottom graph from the 1989 U.S. National Health Interview Survey (Lasser et al., 2000).

Higher Prevalence of Smoking Among 

Patients With Mental Disorders

NIDA Research Report Series 3
Patients with schizophrenia have higher rates of alcohol, tobacco, and 

other drug abuse than the general population. Based on nationally 

representative survey data, 41 percent of respondents with past-month 

mental illnesses are current smokers, which is about double the rate of 

those with no mental illness. In clinical samples, the rate of smoking in 

patients with schizophrenia has ranged as high as 90 percent. 

Various self-medication hypotheses have been proposed to explain the 

strong association between schizophrenia and smoking, although none 

have yet been confirmed. Most of these relate to the nicotine contained 

in tobacco products: Nicotine may help compensate for some of the 

cognitive impairments produced by the disorder and may counteract 

psychotic symptoms or alleviate unpleasant side effects of antipsychotic 

medications. Nicotine or smoking behavior may also help people with 

schizophrenia deal with the anxiety and social stigma of their disease. 

Research on how both nicotine and schizophrenia affect the brain has 

generated other possible explanations for the high rate of smoking 

among people with schizophrenia. The presence of abnormalities in 

particular circuits of the brain may predispose individuals to schizophrenia, 

increase the rewarding effects of drugs like nicotine, or reduce an 

individual’s ability to quit smoking. The involvement of common 

mechanisms is consistent with the observation that both nicotine and 

the medication clozapine (which also acts at nicotine receptors, among 

others) can improve attention and working memory in an animal model 

of schizophrenia. Clozapine is effective in treating individuals with 

schizophrenia. It also reduces their smoking levels. Understanding how 

and why patients with schizophrenia use nicotine is likely to help us 

develop new treatments for both schizophrenia and nicotine dependence.

Smoking and Schizophrenia: SelfMedication or Shared Brain Circuitry?

Common Factors 

Overlapping Genetic Vulnerabilities. A particularly active area 

of comorbidity research involves the 

search for genes that might predispose individuals to develop both addiction and other mental illnesses, 

or to have a greater risk of a second 

disorder occurring after the first 

appears. It is estimated that 40–60 

percent of an individual’s vulnerability to addiction is attributable to 

genetics; most of this vulnerability 

arises from complex interactions 

among multiple genes and from genetic interactions with environmental influences. In some instances, 

a gene product may act directly, 

as when a protein influences how 

a person responds to a drug (e.g., 

whether the drug experience is pleasurable or not) or how long a drug 

remains in the body. But genes can 

also act indirectly by altering how 

an individual responds to stress 

or by increasing the likelihood of 

risk-taking and novelty-seeking behaviors, which could influence the 

development of drug use disorders 

and other mental illnesses. Several 

regions of the human genome have 

been linked to increased risk of both 

drug use disorders and mental illness, including associations with 

greater vulnerability to adolescent 

drug dependence and conduct disorders. 

Involvement of Similar Brain 

Regions. Some areas of the brain 

are affected by both drug use disorders and other mental illnesses. 

For example, the circuits in the 

brain that use the neurotransmitter 

dopamine—a chemical that carries 

messages from one neuron to another—are typically affected by addictive substances and may also be 

involved in depression, schizophrenia, and other psychiatric disorders.

The rate of smoking 

in patients with 

schizophrenia has 

ranged as high as 90 

percent.

4 NIDA Research Report Series
Indeed, some antidepressants 

and essentially all antipsychotic 

medications directly target the 

regulation of dopamine in this 

system, whereas others may have 

indirect effects. Importantly, dopamine pathways have also been implicated in the way in which stress 

can increase vulnerability to drug 

addiction. Stress is also a known 

risk factor for a range of mental 

disorders and therefore provides 

one likely common neurobiological 

link between the disease processes 

of addiction and those of other 

mental disorders. 

The overlap of brain areas involved in both drug use disorders 

and other mental illnesses suggests that brain changes stemming 

from one may affect the other. For 

example, drug abuse that precedes 

the first symptoms of a mental 

illness may produce changes in 

brain structure and function that 

kindle an underlying propensity 

to develop that mental illness. If 

the mental disorder develops first, 

associated changes in brain activity may increase the vulnerability 

to abusing substances by enhancing their positive effects, reducing 

awareness of their negative effects, 

or alleviating the unpleasant effects associated with the mental 

disorder or the medication used to 

treat it.

The Influence of 

Developmental Stage

Adolescence—A Vulnerable 

Time. Although drug abuse and 

addiction can happen at any time 

during a person’s life, drug use

typically starts in adolescence, 

a period when the first signs of 

mental illness commonly appear. 

It is therefore not surprising that 

comorbid disorders can already 

be seen among youth. Significant 

changes in the brain occur during 

adolescence, which may enhance 

vulnerability to drug use and the 

development of addiction and 

other mental disorders. Drugs of 

abuse affect brain circuits involved 

in learning and memory, reward, 

decisionmaking, and behavioral 

control, all of which are still 

maturing into early adulthood. 

Thus, understanding the long-term 

impact of early drug exposure is a 

critical area of comorbidity research. 

Early Occurrence Increases 

Later Risk. Strong evidence has 

emerged showing early drug use to 

be a risk factor for later substance 

abuse problems; additional findings suggest that it may also be a 

risk factor for the later occurrence 

of other mental illnesses. However, this link is not necessarily a 

simple one and may hinge upon 

genetic vulnerability, psychosocial experiences, and/or general 

environmental influences. A 2005 

study highlights this complexity, 

The brain continues to develop into 

adulthood and undergoes dramatic 

changes during adolescence.

One of the brain areas still maturing during adolescence is the prefrontal 

cortex—the part of the brain that enables us to assess situations, make 

sound decisions, and keep our emotions and desires under control. 

The fact that this critical part of an adolescent’s brain is still a work in 

progress puts them at increased risk for poor decisions (such as trying 

drugs or continuing abuse). Thus, introducing drugs while the brain is still 

developing may have profound and long-lasting consequences. 

5 20 AGES

NIDA Research Report Series 5
with the finding that frequent 

marijuana use during adolescence 

can increase the risk of psychosis 

in adulthood, but only in individuals who carry a particular gene 

variant (see sidebar, “The Influence of Adolescent Marijuana Use 

on Adult Psychosis Is Affected by 

Genetic Variables”). 

It is also true that having a 

mental disorder in childhood 

or adolescence can increase the 

risk of later drug abuse problems, as frequently occurs with 

conduct disorder and untreated 

attention-deficit hyperactivity 

disorder (ADHD). This presents a 

challenge when treating children 

with ADHD, since effective treatment often involves prescribing 

stimulant medications with abuse 

potential. This issue has generated 

strong interest from the research 

community, and although the results are not yet conclusive, most 

studies suggest that ADHD medications do not increase the risk of 

drug abuse among children with 

ADHD (see page 2, “Childhood 

ADHD and Later Drug Problems”). 

Regardless of how comorbidity 

develops, it is common in youth 

as well as adults. Given the high 

prevalence of comorbid mental 

disorders and their likely adverse impact on substance abuse 

treatment outcomes, drug abuse 

programs for adolescents should 

include screening and, as needed, 

treatment for comorbid mental 

disorders.

The high rate of comorbidity between drug abuse and addiction 

and other mental disorders argues for a comprehensive 

approach to intervention that identifies and evaluates each 

disorder concurrently, providing treatment as needed.

Percentage of Individuals 

Meeting Diagnostic Criteria for 

Schizophreniform Disorder at Age 26

0%

3

6

9

12

15

Met/Met Val/Met Val/Val

No adolescent cannabis use

Adolescent cannabis use

The Influence of Adolescent 

Marijuana Use on Adult Psychosis 

Is Affected by Genetic Variables

Source: Caspi A, Moffitt TE, Cannon M, et al., 2005.

The above figure shows that variations in a gene can affect the likelihood 

of developing psychosis in adulthood following exposure to cannabis 

in adolescence. The catechol-O-methyltransferase gene regulates 

an enzyme that breaks down dopamine, a brain chemical involved in 

schizophrenia. It comes in two forms: Met and Val. Individuals with 

one or two copies of the Val variant have a higher risk of developing 

schizophrenic-type disorders if they used cannabis during adolescence 

(dark bars). Those with only the Met variant were unaffected by cannabis 

use. These findings hint at the complexity of factors that contribute to 

comorbid conditions.

6 NIDA Research Report Series
How Can 

Comorbidity Be 

Diagnosed?

The high rate of comorbidity 

between drug use disorders and 

other mental illnesses argues 

for a comprehensive approach 

to intervention that identifies 

and evaluates each disorder 

concurrently, providing treatment 

as needed. The needed approach 

calls for broad assessment tools 

that are less likely to result in a 

missed diagnosis. Accordingly, 

patients entering treatment 

for psychiatric illnesses should 

also be screened for substance 

use disorders and vice versa. 

Accurate diagnosis is complicated, 

however, by the similarities 

between drug-related symptoms 

such as withdrawal and those 

of potentially comorbid mental 

disorders. Thus, when people who 

abuse drugs enter treatment, it 

may be necessary to observe them 

after a period of abstinence in 

order to distinguish between the 

effects of substance intoxication 

or withdrawal and the symptoms 

of comorbid mental disorders. 

This practice would allow for a 

more accurate diagnosis and more 

targeted treatment.

How Should 

Comorbid 

Conditions Be 

Treated?

A fundamental principle emerging 

from scientific research is the 

need to treat comorbid conditions 

concurrently—which can be a 

difficult proposition (see page 

9, “Barriers to Comprehensive 

Treatment of Comorbidity”). 

Patients who have both a drug 

use disorder and another mental 

illness often exhibit symptoms that 

are more persistent, severe, and 

resistant to treatment compared 

with patients who have either 

disorder alone. Nevertheless, 

steady progress is being made 

through research on new and 

existing treatment options 

for comorbidity and through 

health services research on 

implementation of appropriate 

screening and treatment within 

a variety of settings, including 

criminal justice systems. 

Medications

Effective medications exist 

for treating opioid, alcohol, 

and nicotine addiction and for 

alleviating the symptoms of 

many other mental disorders, yet 

most have not been well studied 

in comorbid populations. Some 

medications may benefit multiple 

problems. For example, evidence 

suggests that bupropion (trade 

names: Wellbutrin, Zyban), 

approved for treating depression 

and nicotine dependence, might 

also help reduce craving and use 

of the drug methamphetamine. 

Clearly, more research is needed 

to fully understand and assess 

the actions of combined or dually 

effective medications. 

Behavioral Therapies 

Behavioral treatment (alone or in 

combination with medications) 

is the cornerstone to successful 

outcomes for many individuals 

with drug use disorders or other 

mental illnesses. And while 

behavior therapies continue to 

be evaluated for use in comorbid 

populations, several strategies 

have shown promise for treating 

specific comorbid conditions (see 

page 8, “Examples of Promising 

Behavioral Therapies for Patients 

With Comorbid Conditions”). 

Most clinicians and 

researchers agree that broad 

spectrum diagnosis and 

concurrent therapy will lead 

to more positive outcomes 

for patients with comorbid 

conditions. Preliminary findings 

support this notion, but research 

is needed to identify the most 

effective therapies (especially 

studies focused on adolescents).

NIDA Research Report Series 7
Examples of Promising Behavioral Therapies 

for Patients with Comorbid Conditions

Adults

Therapeutic Communities (TCs) 

TCs focus on the “resocialization” of 

the individual and use broad-based 

community programs as active 

components of treatment. TCs are 

particularly well suited to deal with 

criminal justice inmates, individuals 

with vocational deficits, women who 

need special protections from harsh 

social environments, vulnerable 

or neglected youth, and homeless 

individuals. In addition, some evidence 

suggests the utility of incorporating 

TCs for adolescents who have been 

in treatment for substance abuse and 

related problems.

Assertive Community Treatment 

(ACT)

ACT programs integrate the behavioral 

treatment of other severe mental 

disorders, such as schizophrenia, and 

co-occurring substance use disorders. 

ACT is differentiated from other forms 

of case management through factors 

such as a smaller caseload size, team 

management, outreach emphasis, a 

highly individualized approach, and 

an assertive approach to maintaining 

contact with patients. 

Dialectical Behavior Therapy (DBT)

DBT is designed specifically to reduce 

self-harm behaviors (such as selfmutilation and suicidal attempts, 

thoughts, or urges) and drug abuse. 

It is one of the few treatments that 

is effective for individuals who meet 

the criteria for borderline personality 

disorder.

Exposure Therapy

Exposure therapy is a behavioral 

treatment for some anxiety disorders 

(phobias, PTSD) that involves repeated 

exposure to or confrontation with a 

feared situation, object, traumatic event, 

or memory. This exposure can be real, 

visualized, or simulated, and always is 

contained in a controlled therapeutic 

environment. The goal is to desensitize 

patients to the triggering stimuli and help 

them learn to cope, eventually reducing 

or even eliminating symptoms. Several 

studies suggest that exposure therapy 

may be helpful for individuals with 

comorbid PTSD and cocaine addiction, 

although retention in treatment is 

difficult. 

Integrated Group Therapy (IGT)

IGT is a new treatment developed 

specifically for patients with bipolar 

disorder and drug addiction, designed to 

address both problems simultaneously.

Adolescents

Multisystemic Therapy (MST)

MST targets key factors (attitudes, 

family, peer pressure, school and 

neighborhood culture) associated with 

serious antisocial behavior in children 

and adolescents who abuse drugs.

Brief Strategic Family Therapy (BSFT) 

BSFT targets family interactions that 

are thought to maintain or exacerbate 

adolescent drug abuse and other cooccurring problem behaviors. These 

problem behaviors include conduct 

problems at home and at school, 

oppositional behavior, delinquency, 

associating with antisocial peers, 

aggressive and violent behavior, and 

risky sexual behaviors.

Cognitive-Behavioral Therapy (CBT) 

CBT is designed to modify harmful 

beliefs and maladaptive behaviors. CBT 

is the most effective psychotherapy 

for children and adolescents with 

anxiety and mood disorders, and also 

shows strong efficacy for substance 

abusers. (CBT is also effective for 

adult populations suffering from drug 

use disorders and a range of other 

psychiatric problems.) 

8 NIDA Research Report Series
Barriers to Comprehensive 

Treatment of Comorbidity 

Although research supports the need for comprehensive treatment to 

address comorbidity, provision of such treatment can be problematic for a 

number of reasons: 

• In the United States, different treatment systems address drug use 

disorders and other mental illnesses separately. Physicians are most 

often the front line of treatment for mental disorders, whereas drug 

abuse treatment is provided in assorted venues by a mix of health 

care professionals with different backgrounds. Thus, neither system 

may have sufficiently broad expertise to address the full range of 

problems presented by patients. People also use these health care 

systems differently, depending on insurance coverage and social 

factors. For example, when suffering from substance abuse and 

mental illness comorbidities, women more often seek help from 

mental health practitioners, whereas men tend to seek help through 

substance abuse treatment channels. 

• A lingering bias remains in some substance abuse treatment centers 

against using any medications, including those necessary to treat 

serious mental disorders such as depression. Additionally, many 

substance abuse treatment programs do not employ professionals 

qualified to prescribe, dispense, and monitor medications. 

• Many of those needing treatment are in the criminal justice system. 

It is estimated that about 45 percent of offenders in State and 

local prisons and jails have a mental health problem comorbid with 

substance abuse or addiction. However, adequate treatment services 

for both drug use disorders and other mental illnesses are greatly 

lacking within these settings. While treatment provision may be 

burdensome for the criminal justice system, it offers an opportunity to 

positively affect the public’s health and safety. Treatment of comorbid 

disorders can reduce not only associated medical complications, 

but also negative social outcomes by mitigating against a return to 

criminal behavior and reincarceration. 

Exposure to 

Traumatic Events 

Puts People at 

Higher Risk of 

Substance Use 

Disorders

Physically or emotionally traumatized 

people are at much higher risk of 

abusing licit, illicit, and prescription 

drugs. This linkage is of particular 

concern for returning veterans 

since nearly 1 in 5 military 

service members back from Iraq 

and Afghanistan have reported 

symptoms of post-traumatic stress 

disorder (PTSD) or major depression. 

Recent epidemiological studies 

suggest that as many as half of all 

veterans diagnosed with PTSD also 

have a co-occurring substance use 

disorder (SUD), which could pose an 

enormous challenge for our health 

care system. Many PTSD programs 

do not accept individuals with active 

SUDs, and traditional SUD clinics 

defer treatment of trauma-related 

issues. Nevertheless, there are 

treatments at different stages of 

clinical validation for comorbid PTSD 

and SUD; these include various 

combinations of psychosocial (e.g., 

exposure therapy) and pharmacologic 

(e.g., mood stabilizers, anxiolytics, 

and antidepressants) interventions. 

However, research is urgently 

needed to identify the best 

treatment strategies for addressing 

PTSD/SUD comorbidities, and to 

explore whether different treatments 

might be needed in response to 

civilian versus combat PTSD.

NIDA Research Report Series 9
Glossary 

Addiction: A chronic, relapsing 

disease characterized by compulsive 

drug seeking and use and by longlasting changes in the brain. 

Antisocial Personality Disorder: A 

disorder characterized by antisocial 

behaviors that involve pervasive 

disregard for and violation of the 

rights, feelings, and safety of others. 

These behaviors begin in early 

childhood (conduct disorder) or the 

early teenage years and continue into 

adulthood. 

Anxiety Disorders: Varied 

disorders that involve excessive or 

inappropriate feelings of anxiety or 

worry. Examples are panic disorder, 

PTSD, social phobia, and others.

Attention-Deficit Hyperactivity 

Disorder (ADHD): A disorder that 

typically presents in early childhood, 

characterized by inattention, 

hyperactivity, and impulsivity. 

Bipolar Disorder: A mood disorder 

characterized by alternating 

episodes of depression and mania or 

hypomania.

Comorbidity: The occurrence of two 

disorders or illnesses in the same 

person, either at the same time 

(co-occurring comorbid conditions) 

or with a time difference between 

the initial occurrence of one and 

the initial occurrence of the other 

(sequentially comorbid conditions).

Conduct Disorder: A repetitive and 

persistent pattern of behavior in 

children or adolescents in which the 

basic rights of others or major ageappropriate societal norms or rules 

are violated. 

Depression: A disorder marked 

by sadness, inactivity, difficulty 

with thinking and concentration, 

significant increase or decrease 

in appetite and time spent 

sleeping, feelings of dejection and 

hopelessness, and, sometimes, 

suicidal thoughts or an attempt to 

commit suicide. 

Dopamine: A brain chemical, 

classified as a neurotransmitter, 

found in regions of the brain that 

regulate movement, emotion, 

motivation, and pleasure. 

Dual Diagnosis/Mentally Ill 

Chemical Abuser (MICA): Other 

terms used to describe the 

comorbidity of a drug use disorder 

and another mental illness.

Major Depressive Disorder: A mood 

disorder having a clinical course of 

one or more serious depression 

episodes that last 2 or more weeks. 

Episodes are characterized by a loss 

of interest or pleasure in almost all 

activities; disturbances in appetite, 

sleep, or psychomotor functioning; 

a decrease in energy; difficulties in 

thinking or making decisions; loss of 

self-esteem or feelings of guilt; and 

suicidal thoughts or attempts.

Mania: A mood disorder 

characterized by abnormally and 

persistently elevated, expansive, or 

irritable mood; mental and physical 

hyperactivity; and/or disorganization 

of behavior.

Mental Disorder: A mental condition 

marked primarily by sufficient 

disorganization of personality, mind, 

and emotions to seriously impair the 

normal psychological or behavioral 

functioning of the individual. 

Addiction is a mental disorder.

Neurotransmitter: A chemical 

produced by neurons to carry 

messages from one nerve cell to 

another.

Post-Traumatic Stress Disorder 

(PTSD): A disorder that develops 

after exposure to a highly stressful 

event (e.g., wartime combat, 

physical violence, or natural disaster). 

Symptoms include sleeping 

difficulties, hypervigilance, avoiding 

reminders of the event, and reexperiencing the trauma through 

flashbacks or recurrent nightmares.

Psychosis: A mental disorder (e.g., 

schizophrenia) characterized by 

delusional or disordered thinking 

detached from reality; symptoms 

often include hallucinations.

Schizophrenia: A psychotic disorder 

characterized by symptoms that 

fall into two categories: (1) positive 

symptoms, such as distortions in 

thoughts (delusions), perception 

(hallucinations), and language and 

thinking and (2) negative symptoms, 

such as flattened emotional 

responses and decreased goaldirected behavior. 

Self-Medication: The use of a 

substance to lessen the negative 

effects of stress, anxiety, or other 

mental disorders (or side effects 

of their pharmacotherapy). Selfmedication may lead to addiction 

and other drug- or alcohol-related 

problems.

10 NIDA Research Report Series
References

Biederman, J.; Monuteaux, M.C.; 

Spencer, T.; Wilens, T.E.; 

Macpherson, H.A.; and Faraone, 

S.V. Stimulant therapy and risk 

for subsequent substance use 

disorders in male adults with 

ADHD: A naturalistic controlled 

10-year follow up study. Am J 

Psychiatry 165(5):597–603, 2008.

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NIDA Research Report Series 11
Where can I get more scientific 

information on comorbid addiction 

and other mental illnesses?

To learn more about drug abuse 

and other mental illnesses, contact....

Where Can I Get More Scientific Information on 

Comorbid Addiction and Other Mental Illnesses?

To learn more about drug use disorders and other 

mental illnesses, or to order materials on these 

topics free of charge in English or Spanish, visit the 

NIDA Web site at www.drugabuse.gov or contact 

the DrugPubs Research Dissemination Center at 

877-NIDA-NIH (877-643-2644; 

TTY/TDD: 240-645-0228).

What’s New on the NIDA Web Site

• Information on drugs of abuse 

• Publications and communications (including NIDA 

Notes and Addiction Science & Clinical Practice

journal) 

• Calendar of events 

• Links to NIDA organizational units 

• Funding information (including program

announcements and deadlines) 

• International activities 

• Links to related Web sites (access to Web sites of 

many other organizations in the field)

NIDA Web Sites

drugabuse.gov 

backtoschool.drugabuse.gov 

smoking.drugabuse.gov 

hiv.drugabuse.gov 

marijuana-info.org 

clubdrugs.gov 

steroidabuse.gov 

teens.drugabuse.gov 

inhalants.drugabuse.gov

Other Web Sites

Information on drug abuse and other mental illnesses 

is also available through these other Web sites:

• National Institute of Mental Health: www.nimh.

nih.gov 

• National Institute on Alcohol Abuse and 

Alcoholism: www.niaaa.nih.gov 

• Substance Abuse and Mental Health Services 

Administration Health Information Network:

www.samhsa.gov/shin

NIH Publication Number 10-5771

Printed December 2008, Revised September 2010

Feel free to reprint this publication.