Effect of cannabis use on the course of schizophrenia in male patients: A prospective cohort study

Abstract Background Findings on the impact of cannabis use on the course of schizophrenia are inconsistent and not conclusive. Aims To study the effect of cannabis use on the course of schizophrenia taking into account the effects of the quantity of cannabis use and important confounders. Methods Prospective cohort study with assessments of symptoms, confounders and hospitalizations at baseline, 6 month and 12 month follow up. Results In a representative cohort of 145 male patients with schizophrenia, 68 (46.9%) used cannabis. Mean age at onset of schizophrenia in cannabis using patients was significantly lower than in non-cannabis using patients. No other cross-sectional demographic or clinical differences were observed between users and non-users. In a series of longitudinal analyses, cannabis use was not associated with differences in psychopathology, but relapse in terms of the number of hospitalizations was significantly higher in cannabis using patients compared to non-cannabis using patients. Conclusions Patients with schizophrenia using cannabis are more frequently hospitalized than non-cannabis using patients but do not differ with respect to psychopathology. Possible explanations for these findings are discussed.