The link between cannabis use and the development of psychosis is established. This systematic review and meta-analysis of 24 studies summarizes the evidence of the impact of cannabis use after the onset of psychosis (schizophrenia, schizoaffective, or bipolar if outcome was reported as number of psychotic episodes). Researchers compared the outcomes of those who continued using cannabis, those who stopped, and those who did not have cannabis use.
- Irrespective of the stage of the psychotic disorder, continued cannabis use was associated with a greater risk of recurrence of psychosis, compared with people without cannabis use (Cohen’s d=0.36) and those who discontinued use (d=0.28).
- Those who continued cannabis use had longer hospital admissions compared with those without cannabis use (d=0.36). People with cannabis use spent an additional 8.5 days per year in the hospital (for psychosis symptoms).
- Relapse to psychosis did not differ between those who discontinued cannabis and those without cannabis use (d=0.02).
- Continued cannabis use predicted severity of positive psychotic symptoms (e.g., hallucinations, delusions, disorganization), but not negative symptoms (e.g., absent or blunted emotional response, monotone speech, difficulty initiating tasks or thinking, anhedonia, apathy, social withdrawal). Those who discontinued cannabis use showed a better level of functioning.