There is increasing evidence that cannabis use can act as a risk factor for the development of bipolar as well as psychotic disorders. A recent systematic review by Gibbs and colleagues identified several studies supporting a link between cannabis use and subsequent relapse of manic symptoms. This review also included a meta-analysis of two large prospective cohort studies, which found that cannabis use almost trebled the risk of new-onset subthreshold manic symptoms after adjusting for potential confounding factors. A further large prospective cohort study found cannabis use increased the risk of first episode bipolar by a factor of 5 after adjusting for confounders, and demonstrated evidence of a dose–response relationship.
Source: wiktionary