15 - Does cannabis use cause schizophrenia? The epidemiological evidence

Summary

This chapter examines the strength of the epidemiological evidence to support a causal relationship between cannabis use and increased risk of schizophrenia and other psychotic disorders. There are limited data examining the relationship between cannabis use and psychotic outcomes in longitudinal studies. As most of the studies excluded subjects with psychosis at baseline, or adjusted for psychotic symptoms at baseline, the observed associations are unlikely to reflect reverse causation. Three studies have examined whether there is a synergistic effect of cannabis use and childhood trauma in increasing the risk of psychotic outcomes. The studies report that the presence of both childhood trauma and cannabis use significantly increase the absolute risk of psychotic outcomes beyond the risk posed by either risk factor alone. If regular use of cannabis doubles the risk of developing schizophrenia, as suggested by some studies, the individual lifetime risk in such individuals would increase to about 1.5%.