Commentary on Ostergaard et al. (2017): Evidence of an association between cannabis use and suicide in subjects with bipolar disorder Comorbid cannabis use disorder may affect negatively the long-term outcomes of bipolar disorder. Although the direction of the association between cannabis use and bipolar symptoms needs to be clarified, there is now sufficient evidence to hypothesize a role of cannabis use in increasing the risk of suicide.The Danish nation-wide, prospective, register-based study by Østergaard et al. [1] tested associations between different substance use disorders and attempted or completed suicides in a large sample of individuals with severe mental illness (n = 35 625 with schizophrenia; n = 9279 with bipolar disorder; n = 72 530 with depression; and n = 63 958 with personality disorder). The authors found that substance use disorders were strongly associated with risk of completed suicides and suicide attempts. The detrimental effect of co-occurring substance use disorders on mental disorder course was thus confirmed [2]. In particular, individuals with comorbid bipolar and substance use disorders were more likely to have a history of suicide attempt compared with their non-comorbid counterparts (23.2 versus 10.2%) [1]. Results of this study are consistent with previous meta-analytic data [3] showing that rates of suicide attempts among subjects with bipolar and alcohol or substance use disorders are twofold higher than in subjects with bipolar disorder only.The relationship between cannabis and suicide in subjects with bipolar disorder is, however, worth further discussion, given that previous research exploring this topic has produced mixed results [3,4]. For example, it has been reported that approximately one-fifth of subjects with bipolar disorder, especially males with an earlier age of illness onset, misuse cannabis [5]. Cannabis use is associated with increased manic, depressive and psychotic symptoms [6,7], rapid cycling [8], low compliance [7] and poorer functioning [9], therefore affecting long-term clinical outcomes and decreasing the probability of remission [10]. Previous meta-analytic data [3,11], including four different studies [12][13][14][15] based on 3439 subjects with bipolar disorder (559 with comorbid cannabis use disorder), have shown a significant association [odds ratio (OR) = 1.44; 95% confidence interval (CI) = 1.07-1.94] between suicide attempts and cannabis use. Nevertheless, the small effect size, the wide confidence intervals and a partial inconsistency of results (a moderate heterogeneity across studies) all limit confidence in these findings. Moreover, the cross-sectional design of this meta-analysis has not permitted causal inferences of the relationship between suicide attempts and cannabis use disorder to be considered.Longitudinal analyses are now needed to explore the potential role of cannabis in increasing the risk of suicide among individuals with bipolar disorder. Findings from the prospective study by Østergaard et al. [1] showed that current canna...