Research suggests that cannabis use negatively impacts on onset and outcome of schizophrenia. Possible effects in mood disorders have received little investigation. The first study analysing the influence of cannabis exposure on clinical and social treatment outcomes within a bipolar disorder (BP) population during 1 year of treatment is presented.

3684 patients were enrolled in an observational study when psychotropic treatment for mania was initiated/changed. The influence of cannabis exposure on baseline-corrected clinical and social treatment outcome measures was examined. Mediating effects of six variables on associations between cannabis and outcome measures were investigated further.

Over 12 months of treatment, cannabis users exhibited higher levels of BP overall illness severity, mania and psychosis, and less severe depression symptoms compared to non-users. These associations were most frequently mediated by abuse of alcohol and other substances. Users more frequently abused alcohol and other substances; these associations were not mediated by other variables. Cannabis users engaged in more social activities but had a higher probability of not having a relationship and fewer dependents to care for. Associations with activities and dependents to care for were mediated by various variables, whereas no variables mediated the association with not having a relationship.

Cannabis use impacts on clinical outcomes in patients with BP, with a modest impact on social outcomes. More research is required to further elucidate the mechanism by which cannabis exerts its influence. Understanding the associations between cannabis use and outcome measures may offer valuable insights into treatment strategies.