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.

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 risky.

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:

• 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.
• Mental illnesses can lead to drug abuse. Individuals with overt, mild, or even subclinical mental disorders may abuse drugs as a form of self-medication. For example, the use of tobacco products by patients with schizophrenia is believed to lessen the symptoms of the disease and improve cognition.
• 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.

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.

National Institute on Drug Abuse – Comorbidity: Addiction and Other Mental Illnesses