Cannabis Use in Patients at Clinical High Risk of Psychosis: Impact on Prodromal Symptoms and Transition to Psychosis

Background: The relation between cannabis use and psychotic disorders has been investigated extensively. A series of metaanalytic reviews reveal a robust association between cannabis use and the development of psychosis and schizophrenia. However, the actual impact of cannabis use in subjects at high clinical risk for psychosis (CHR) is still unclear. Method: We conducted a systematic review of publications measuring the impact of cannabis use on CHR symptomatology and transition to a first psychotic episode. Results: Of 729 potentially relevant papers, 11 met inclusion criteria. The results of these studies were mixed. In some studies, cannabis use was associated with more severe symptoms at baseline, increased pre-psychotic symptoms immediately after intoxication, and earlier onset of certain high-risk symptoms. In others, no significant association between cannabis use and baseline symptomatology was found. In one study, cannabis use was even significantly associated with a decrease in pre-psychotic negative symptoms, and with fewer symptoms of depression and anxiety. Four out of 5 studies reported no significant effect of cannabis use on transition to psychosis. Conclusions: Cannabis use seems to provoke and enhance subclinical symptoms in CHR subjects. However, the results provide no consistent evidence for an association between cannabis use and transition to a first psychosis in CHR subjects.