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.*

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.

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

* 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.