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NIDA Home > Research Report Series > Comorbidity: Addiction and Other Mental Illnesses

Research Report Series
Addiction and Other Mental Illnesses

Letter from the Director

drawing of a brain
What is Comorbidity?
When two disorders or illnesses occur in the same person, simultaneously or sequentially, they are called comorbid. Comorbidity also implies interactions between the illnesses that affect the course and prognosis of both.

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, and so it remains a research priority for NIDA. This Research Report provides information on the state of the science in this area. And 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 various 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. Even though 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 success. 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.
National Institute on Drug Abuse

* Drug abuse and drug dependence, or addiction, are considered drug use disorders–a subgroup of mental disorders–when they meet the diagnostic criteria delineated in the Diagnostic and Statistical Manual of Mental Disorders (DSM). Drug dependence, as DSM defines it, is synonymous with the term "addiction," which will be used preferentially in this report. Since the focus of this report is on comorbid drug use disorders and other mental illnesses, the terms "mental illness"/"mental disorders" will refer here to disorders other than substance use, 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.

This report is also available in PDF format, Comorbidity, [PDF format, 1.1 MB]
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All materials appearing in the Research Reports Series are in the public domain and may be reproduced without permission from NIDA. Citation of the source is appreciated.

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