The relationship between cannabis and schizophrenia is complex, and our understanding is limited as to why mental health of some individuals is compromised by cannabis use whereas for others there appears to be no adverse effect.[i] Studies among patients with schizophrenia have been contradictory. There is a concern that exposure to cannabis during adolescence is a risk factor for psychosis,142 and cannabis use can exacerbate symptoms in some individuals who already have schizophrenia.193
A 2012 meta-analysis demonstrated that regular cannabis use was associated with better cognitive performance in both established schizophrenia and first episode psychosis patients (FEP) compared to non-using patients.[ii] It is not known if this association is due to a positive effect of cannabis on schizophrenia, or if some of these patients developed psychosis after a relatively early initiation into cannabis use. This study also concluded that cannabis use was associated with a younger age of psychosis onset and more positive symptoms.194
A 2017 meta-analysis reports that high levels of cannabis use increases the risk of psychotic outcome and confirms a dose-response relationship, but a causal link has not been clearly established.[iii]
While cannabis appears to worsen psychotic symptoms in schizophrenia, a number of studies show that CBD has anti-psychotic effects. A 2020 systematic review concludes that there is limited evidence of antipsychotic efficacy, but that no improvement in cognition or functioning was found.[iv]
[i] Hamilton I. Cannabis, psychosis and schizophrenia: unravelling a complex interaction. Addiction. 112(9), 1653-1657 (2017). doi: 10.1111/add.13826
[ii] Yücel, M., Bora, E., Lubman, D.I., Solowij, N., Brewer, W.J., Cotton, S.M., Conus, P., Takagi, M.J., Fornito, A., Wood, S.J., McGorry, P.D., Pantelis, C. The impact of cannabis use on cognitive functioning in patients with schizophrenia: a meta-analysis of existing findings and new data in a first-episode sample. Schizophr Bull. 38(2), 316-30 (2012). doi: 10.1093/schbul/sbq079
[iii] Marconi, A., Di Forti, M., Lewis, C.M., Murray, R.M., Vassos, E. Meta-analysis of the Association Between the Level of Cannabis Use and Risk of Psychosis. Schizophr Bull. 42(5), 1262-9 (2016). doi: 10.1093/schbul/sbw003
[iv] Ghabrash, M. F. et al. Cannabidiol for the treatment of psychosis among patients with schizophrenia and other primary psychotic disorders: A systematic review with a risk of bias assessment. Psychiatry Res. 286, 112890 (2020).