As many as 6 in 10 substance abusers also have at least one other mental disorder. Research increasingly supports the benefit of studying and treating co-occurring disorders together, with both medication and behavioral therapies.

Tobacco smoking patterns highlight the striking relationship between addiction and mental illness. Mentally ill individuals are about twice as likely to smoke as others; although they comprise an estimated 28 percent of the population, they consume about 44 percent of all cigarettes smoked, according to a recent U.S. study. Smoking rates are particularly high—75 to 95 percent—among people with schizophrenia. Combining bupropion for nicotine addiction with tailored behavioral smoking cessation treatment can curb smoking by patients with schizophrenia, and controlling symptoms of schizophrenia helps reduce smoking intensity and nicotine addiction.

The reasons why addiction and other mental disorders coincide so frequently are not fully understood. Epidemiological research suggests that each can contribute to the development of the other. Children and adolescents with psychiatric conditions—including conduct disorders, attention deficit hyperactivity disorder (ADHD), and learning disabilities—are at higher risk of abusing drugs than other youth. There is evidence that drug abuse early in life may increase the risk of psychiatric disorders or accelerate their course. NIDA-supported investigators are using neuroimaging, genotyping, statistical modeling, and other tools to parse the interplay of risk factors in the development of such disorders.

Research has yielded some practical help and promises more. Several studies demonstrate that early diagnosis and treatment of ADHD cuts the risk for substance abuse in adolescence, and NIDA is funding research to determine whether early treatment of other psychiatric disorders can have similar benefits. Addressing substance abuse early can prevent the onset or improve the outcome of psychiatric disorders as well. Tailored versions of proven behavioral treatments—for example, cognitive-behavioral and motivational enhancement therapy—are available for clinicians who work with substance abusers with psychiatric conditions.

NIDA's Clinical Trials Network has begun recruiting patients in a large study to test whether treating ADHD can improve substance abuse outcomes in adults and adolescents with both conditions. We are also supporting studies on integrated treatments for addiction and posttraumatic stress disorder (PTSD) and on the relationship between stress-related conditions and drug abuse.