Etiology
Etiological models have been advanced to describe the observed association and offer theoretical guidance for all permutations of directionality between substance use and internalizing psychopathology. Most notably, self-medication models posit that even prior to physiological dependence, individuals initially seek and consume substances to mitigate the physical, psychological, and social distress resulting from mental illness (Khantzian, 1985). Alternatively, the psychopharmacological and generally disruptive effects of sustained, problematic substance use may be implicated in the development and exacerbation of psychopathology (e.g. Friedman et al., 1987). There is evidence to suggest that, once present, substance use disorders and mental illness may mutually influence each other (e.g. Price et al., 2004). The co-occurrence of substance use and internalizing disorders may also be due to an underlying vulnerability to both conditions.