Does change in cannabis use in established psychosis affect clinical outcome?

BACKGROUND: Cannabis use has been identified as a potent predictor of the earlier onset of psychosis, but meta-analysis has not indicated that it has a clear effect in established psychosis. AIM: To assess the association between cannabis and outcomes, including whether change in cannabis use affects symptoms and functioning, in a large sample of people with established nonaffective psychosis and comorbid substance misuse. METHODS: One hundred and sixty participants whose substance use included cannabis were compared with other substance users (n = 167) on baseline demographic, clinical, and substance use variables. The cannabis using subgroup was examined prospectively with repeated measures of substance use and psychopathology at baseline, 12 months, and 24 months. We used generalized estimating equation models to estimate the effects of cannabis dose on subsequent clinical outcomes and whether change in cannabis use was associated with change in outcomes. RESULTS: Cannabis users showed cross-sectional differences from other substances users but not in terms of positive symptoms. Second, cannabis dose was not associated with subsequent severity of positive symptoms and change in cannabis dose did not predict change in positive symptom severity, even when patients became abstinent. However, greater cannabis exposure was associated with worse functioning, albeit with a small effect size. CONCLUSIONS: We did not find evidence of an association between cannabis dose and psychotic symptoms, although greater cannabis dose was associated with worse psychosocial functioning, albeit with small effect size. It would seem that within this population, not everyone will demonstrate durable symptomatic improvements from reducing cannabis.