other illnesses, particularly psychiatric disorders. The Epidemiologic Catchment Area Program sponsored by NIMH documented the high co-occurrence of alcohol and illicit drug use disorders and other psychiatric disorders (Regier et al., 1990). The NCS (see above) provides the first nationally representative estimates on the co-occurrence of drug abuse and psychiatric disorders. The principal finding is that there is a significant overlap in diagnoses. Roughly half of those age 15-54 who had a lifetime addictive disorder (as defined by DSM-III-R) also had a lifetime psychiatric disorder. Similarly, about half of those with a lifetime psychiatric disorder also had a lifetime addictive disorder. The great majority (84 percent) of those with lifetime co-occurrence reported that their first psychiatric disorder occurred prior to their first addictive disorder (Kessler et al., 1996). Further, the NCS found that all the psychiatric disorders (identified) are consistently more strongly associated with dependence as opposed to abuse. Compared to other affective disorders and anxiety disorders, mania is more strongly associated with dependence on alcohol or illicit drugs. Conduct disorders and adult antisocial behavior were more strongly associated with both abuse and dependence than the anxiety disorders or any of the affective disorders other than mania (Kessler et al., 1996). Those extremely high rates of co-occurrence obviously have great implication for treatment, prevention, and understanding the nature of addictive disorders (Chapters 5 and 8).

Although the NCS studied comorbidities with psychiatric disorders, addictive disorders also co-occur with other health and physical disorders. For example, tuberculosis, hepatitis, sexually transmitted diseases, and AIDS are all more prevalent among alcoholics or illicit drug abusers than nonabusers. Other conditions such as homelessness and victimization may also be co-occurring, but await further epidemiological study.

As with any epidemiological study, the fact of co-occurrence does not indicate a causal connection. There are a variety of possible causal relationships. An important task for future research is to provide analysis of the possible reasons for co-occurring disorders in an effort to improve treatment and prevention strategies.

Natural History of Drug Use

Epidemiological research also has contributed to knowledge of the natural history, or life course, of drug use and abuse in the general population, including delineation of stages of involvement with different drugs and degrees of involvement with different types of drugs. Such natural histories require longitudinal studies of individuals over time to discern changes with age; however, there are relatively few studies that span a broad age range. Moreover, it is preferable to utilize a cohort sequential



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