Objective: We sought to examine the relation between cannabis abuse and the symptomatic course of recent-onset schizophrenia and related disorders.
Design: A prospective cohort study over a year using monthly Brief Psychiatric Rating Scale assessments.
Participants: Cannabis-abusing patients (n = 24) were compared with nonabusers (n = 69). Eleven patients were mild and 13 were heavy cannabis-abusing patients.
Results: Significantly more and earlier psychotic relapses occurred in the cannabis-abusing group (P = .03). This association became stronger when mild and heavy cannabis abuse were distinguished (P = .002). No confounding effect of other variables, eg, other street drugs, was found. In all but one patient, cannabis abuse preceded the onset of the first psychotic symptoms for at least 1 year.
Conclusions: Cannabis abuse and particularly heavy abuse can be considered a stressor eliciting relapse in patients with schizophrenia and related disorders and possibly a premorbid precipitant.