Cannabis Use Disorder and Bipolar II Disorder Share a Familial Risk Factor

CUD in probands was associated with increased CUD in relatives (aOR 2.64). Bipolar II (but not bipolar I or major depression) was also associated with CUD in relatives (aOR 2.57). Among relatives, CUD was associated with bipolar II (aOR 4.50) and major depression (aOR 3.64). Mood episodes typically preceded CUD onset.

Community-based family study in the Washington, DC area (2004-2020). 586 adult probands and 698 first-degree relatives underwent semistructured diagnostic interviews. Mixed-effects models estimated familial aggregation of CUD with mood disorders, adjusting for demographics and comorbidities.

The specific link with bipolar II (but not bipolar I) is new and clinically significant. If a shared genetic vulnerability underlies both conditions, then treating bipolar II early might actually help prevent cannabis use disorder.

This adds genetic evidence to the long-debated question of whether cannabis causes mental illness or whether shared vulnerabilities drive both. The finding that mood episodes typically come first, combined with the family clustering, suggests a shared diathesis model where underlying vulnerability leads to both conditions.

Cross-sectional design cannot definitively establish temporal ordering. The DC metro sample may not represent all populations. Lifetime diagnoses may be subject to recall bias. The relatively small number of CUD cases (55 probands, 68 relatives) limits statistical power for subgroup analyses.