Cannabis use and adherence to antipsychotic medication: a systematic review and meta-analysis

Background: Substance use may increase the risk of non-adherence to antipsychotics, resulting in negative outcomes in patients with psychosis. Method: We aimed to quantitatively summarize evidence regarding the effect of cannabis use, the most commonly used illicit drug amongst those with psychosis, on adherence to antipsychotic medication. Studies were identified through a systematic database search. Adopting random-effects models, pooled odds ratios (OR) for risk of non-adherence to antipsychotic medications were calculated comparing: cannabis-users at baseline v. non-users at baseline; non users v. continued cannabis users at follow-up; non-users v. former users at follow-up; former users v. current users. Results: Fifteen observational studies (n = 3678) were included. Increased risk of non-adherence was observed for cannabis users compared to non-users (OR 2.46, n = 3055). At follow-up, increased risk of non-adherence was observed for current users compared to non-users (OR 5.79, n = 175) and former users (OR 5.5, n = 192), while there was no difference between former users and non-users (OR 1.12, n = 187). Conclusions: Cannabis use increases the risk of non-adherence and quitting cannabis use may help adherence to antipsychotics. Thus, cannabis use may represent a potential target for intervention to improve medication adherence in those with psychosis.