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Several large-scale analyses have associated cannabis use with a risk of developing psychosis or schizophrenia.[Semple et al., 2005; Vaucher et al., 2017] The association is still somewhat controversial, and it is hard to be certain of an association that can only be demonstrated using observational or epidemiological studies.[Vaucher et al., 2017] A recent analysis found little support for an association between cannabis use and the development of schizophrenia after adjusting for certain appropriate covariates.[Ryan et al., 2019] A randomised-controlled trial (RCT) is often considered the ‘gold standard’ to demonstrate an association, however, in the case of cannabis and schizophrenia, an RCT would be highly unethical![Vaucher et al., 2017]
Questions remain over the salience of the association between cannabis use and schizophrenia; in Australia, for example, the use of cannabis is much higher than elsewhere, but the incidence of schizophrenia is not.[Semple et al., 2005] What is the reason for this discrepancy? There are also questions surrounding the importance of early exposure to cannabis – is exposure in adolescence necessary for cannabis to exert its effect as a risk factor for psychosis?[Semple et al., 2005] More research is needed to answer these questions.[Semple et al., 2005]
References:
Ryan JE, Veliz P, McCabe SE, et al. Association of early onset of cannabis, cigarette, other drug use and schizophrenia or psychosis. Schizophr Res 2019.[Epub]
Semple DM, McIntosh AM, Lawrie SM. Cannabis as a risk factor for psychosis: systematic review. J Psychopharmacol 2005; 19 (2): 187–194.
Vaucher J, Keating BJ, Lasserre AM, et al. Cannabis use and risk of schizophrenia: a mendelian randomization study. Mol Psychiatry 2018; 23 (5): 1287–1292.