Parsing the association between bipolar, conduct, and substance use disorders: a familial risk analysis.

BACKGROUND Bipolar disorder has emerged as a risk factor for substance use disorders (alcohol or drug abuse or dependence) in youth; however, the association between bipolar disorder and substance use disorders is complicated by comorbidity with conduct disorder. We used familial risk analysis to disentangle the association between the three disorders. METHODS We compared relatives of four proband groups: 1) conduct disorder + bipolar disorder, 2) bipolar disorder without conduct disorder, 3) conduct disorder without bipolar disorder, and 4) control subjects without bipolar disorder or conduct disorder. All subjects were evaluated with structured diagnostic interviews. For the analysis of substance use disorders, Cox proportional hazard survival models were utilized to compare age-at-onset distributions. RESULTS Bipolar disorder in probands was a risk factor for both drug and alcohol addiction in relatives, independent of conduct disorder in probands, which was a risk factor for alcohol dependence in relatives independent of bipolar disorder in probands, but not for drug dependence. The effects of bipolar disorder and conduct disorder in probands combined additively to predict the risk for substance use disorders in relatives. CONCLUSIONS The combination of conduct disorder + bipolar disorder in youth predicts especially high rates of substance use disorders in relatives. These findings support previous results documenting that when bipolar disorder and conduct disorder occur comorbidly, both are validly diagnosed disorders.