Treating Cannabis Use in Schizophrenia and Other Psychotic Disorders

As is abundantly clear from other chapters in this book, cannabis misuse is a serious problem in people with schizophrenia and psychotic disorders, with rates of problematic cannabis use (e.g., cannabis use disorder) ~25% compared to ~3% in the general population. Cannabis misuse in people with schizophrenia is associated with symptom exacerbation, acute hospitalization, poorer psychosocial outcomes, and reduced quality-of-life. The mechanisms for this common co-morbidity are poorly understood, but may relate to self-medication, shared biological and/or genetic vulnerability, and social and environmental factors. Moreover, the potency of the cannabis consumed, along with dose and frequency of use, may contribute to psychosis risk and poorer clinical course. Reducing the burden of cannabis use in these patients depends on effective engagement and developing novel intervention targets. Behavioural treatments studied to date suggest modest evidence supporting motivational, cognitive-behavioural, and contingency management interventions in reducing cannabis use in psychotic patients. Moreover, amongst pharmacological interventions studied, clozapine may reduce cannabis use in people with schizophrenia, with lesser evidence for other second-generation anti-psychotic drugs, such as olanzapine and ziprasidone. Use of long-acting injectable antipsychotics (LAIs) might be relevant in people whose adherence is poor. Novel treatments such as brain stimulation and smart-phone/web-based interventions should be rigorously evaluated in future studies.