Studies in SUD and psychiatric treatment-seeking populations (McLellan & Druley, 1977; Ross, Glaser, & Germanson, 1988; Rounsaville et al., 1991) have suggested high prevalence rates of co-occurring SUDs and psychiatric disorders. However, treatment-seeking samples may not be representative of community populations, since they tend to have higher rates of comorbidity and may have more severe manifestations of the disorder for which they are seeking treatment. Thus, epidemiological studies of prevalence rates in community populations are important in assessing the true comorbidity prevalence rate.

The two largest U.S. psychiatric epidemiological studies to date, the Epidemiologic Catchment Area (ECA) study (Regier et al., 1990) and the more recent National Comorbidity Survey (NCS; Kessler et al., 1996) demonstrate that co-occurring SUDs and psychiatric disorders are prevalent in community populations. Methodological advancements of the NCS included an expanded scope of the community sample (e.g., the ECA sampled from within five U.S. communities, whereas the NCS sampled nationally representative households), and an advanced version of the Diagnostic and Statistical Manual of Mental Disorders (i.e., DSM-III-R [American Psychiatric Association, 1987]). Also, while both studies surveyed most of the more common psychiatric disorders, the ECA did not include posttraumatic stress disorder (PTSD), whereas the NCS did. Neither epidemiological survey included Axis II disorders other than antisocial personality disorder (ASPD). Despite these limitations and differences between the two studies, their results were often qualitatively similar, although the magnitude of their estimates differed somewhat at times. Among persons with psychiatric disorders, the ECA estimated that 30% had a co-occurring SUD. The prevalence varied by diagnosis, however; co-occurring SUDs were most common in individuals with ASPD, followed by those with bipolar I disorder. In SUD populations, the ECA and the NCS estimated that over half will experience Axis I or II psychiatric disorders in their lifetime. These lifetime estimates do not merely reflect rare or historical periods in an individual's history; the 12-month comorbidity prevalence rate of these disorders was also quite high. For example, the NCS estimated that over 33% of those with bipolar disorder would experience an SUD within 12 months, followed by nearly 20% of those with major depression and 15% of those with an anxiety disorder.

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