The high rate at which mental and substance-use disorders occur together has been well documented in epidemiological and clinical studies (Regier and Farmer 1990; Kessler, Nelson et al. 1996). For example, in the National Comorbidity Study, 51 percent of those who met criteria for a substance disorder at some time in their life also met criteria for a mental disorder at some point, and in the large majority of cases individuals reported that the mental disorder preceded the substance disorder (Kessler et al. 1996). Researchers and clinicians have advanced a number of theories to explain the high rates of co-occurrence. One prominent explanation for the high rates of co-occurrence is that individuals use psychoactive substances to “self-medicate” painful or disturbing psychiatric symptoms (Khantzian 1997; Chilcoat and Breslau 1998; Strakowski and DelBello 2000). Other theories suggest that substance-use disorders cause mental health problems or that substance use and mental health problems have common underlying genetic and environmental causes (Chilcoat and Breslau 1998).

Understanding the underlying causes of co-occurrence is important for improving the treatment and prevention of mental health and substance-use problems. If self-medication is common, then timely screening and treatment of mental health problems may prove the key in preventing the onset of substance-use disorders among the population with mental disorders. Although predictions about the substitutability of psychoactive substances and mental health care are implicit in the self-medication hypothesis, they have gone unexplored in the health services literature.