People with severe mental illness such as schizophrenia or bipolar disorder have a higher risk for substance use, especially cigarette smoking, and protective factors usually associated with lower rates of substance use do not exist in severe mental illness, according to a new study funded by the National Institute on Drug Abuse (NIDA), part of the National Institutes of Health.

Estimates based on past studies suggest that people diagnosed with mood or anxiety disorders are about twice as likely as the general population to also suffer from a substance use disorder. Statistics from the 2012 National Survey on Drug Use and Health indicate close to 8.4 million adults in the United States have both a mental and substance use disorder. However, only 7.9 percent of people receive treatment for both conditions, and 53.7 percent receive no treatment at all, the statistics indicate.

Studies exploring the link between substance use disorders and other mental illnesses have typically not included people with severe psychotic illnesses.

“Drug use impacts many of the same brain circuits that are disrupted in severe mental disorders such as schizophrenia,” said NIDA Director Dr. Nora D. Volkow. “While we cannot always prove a connection or causality, we do know that certain mental disorders are risk factors for subsequent substance use disorders, and vice versa.”

In the current study, 9,142 people diagnosed with schizophrenia, schizoaffective disorder, or bipolar disorder with psychotic features, and 10,195 controls matched to participants according to geographic region, were selected using the Genomic Psychiatry Cohort program. Mental disorder diagnoses were confirmed using the Diagnostic Interview for Psychosis and Affective Disorder (DI-PAD), and controls were screened to verify the absence of schizophrenia or bipolar disorder in themselves or close family members. The DI-PAD was also used for all participants to determine substance use rates.

Compared to controls, people with severe mental illness were about 4 times more likely to be heavy alcohol users (four or more drinks per day); 3.5 times more likely to use marijuana regularly (21 times per year); and 4.6 times more likely to use other drugs at least 10 times in their lives. The greatest increases were seen with tobacco, with patients with severe mental illness 5.1 times more likely to be daily smokers. This is of concern because smoking is the leading cause of preventable death in the United States.

In addition, certain protective factors often associated with belonging to certain racial or ethnic groups – or being female – did not exist in participants with severe mental illness. “In the general population, women have lower substance use rates than men, and Asian-Americans have lower substance use rates than white Americans, but we do not see these differences among people with severe mental illness,” said Dr. Sarah Hartz, from the Washington University School of Medicine in St. Louis and first author on the study. “We also saw that among young people with severe mental illness, the smoking rates were as high as smoking rates in middle-aged adults, despite success in lowering smoking rates for young people in the general population.”

Previous research has shown that people with schizophrenia have a shorter life expectancy than the general population, and chronic cigarette smoking has been suggested as a major contributing factor to higher morbidity and mortality from malignancy as well as cardiovascular and respiratory diseases. These new findings indicate that the rates of substance use in people with severe psychosis may be underestimated, highlighting the need to improve the understanding of the association between substance use and psychotic disorders so that both conditions can be treated effectively.