Comorbidity potentially has implications for theories of aetiology, prevention and treatment of mental health problems.

Importance for theory

If mental health problems are more likely to occur among those with substance use disorders, this raises important questions about the aetiology of mental disorders (and vice versa). Several hypotheses exist concerning the reasons why comorbidity might occur, including that: (a) there is a causal relationship between the two; (b) that common factors increase the likelihood of both disorders; and (c) that the relationship is spurious (artefactual), resulting from factors such as the methods with which the sample was selected (Caron & Rutter, 1991; Kessler, 1995; Mueser, Drake, & Wallach, 1998). Before we can begin to unravel the reasons behind any 'comorbidity', we need to carefully document the nature of any associations. This will give some insight into possible mechanisms underlying the association.

Importance for treatment

If people who are problematic substance users are more likely to have other mental health problems, this needs to be taken into account both in the assessment of a client, and in determining the most appropriate treatment. Comorbidity is particularly relevant if co-occurring disorders predict a differential clinical outcome, which has been suggested by previous research (e.g. Carey, Carey, & Meisler, 1991; Haywood et al., 1995; Pristach & Smith, 1990; Rouillon, 1996). Attention to comorbid problems may also improve treatment outcome. The efficacy of treatment for alcohol and nicotine dependence, for example, may be improved if treatment for depression is also provided (Hall et al., 1998; Lynskey, 1998).

Importance for prevention

Prevention programs have traditionally operated in isolation from each other. For example, it is often the case that programs addressing suicide prevention, substance use prevention and sex education/sexual risk taking occur separately. There is rarely an attempt to conduct programs aimed at addressing multiple problems in an integrated fashion. Furthermore, there is an increased interest in psychiatry on prevention. The concept of comorbidity has two broad implications for prevention: a) if 'comorbidity' is real, then prevention efforts should be broad in their target; and b) an understanding of the nature of comorbidity will help dictate the targets of prevention. If comorbidity arises because different problems or disorders share the same risk factors, then interventions addressing these risk factors should reduce the prevalence of these multiple problems.