These historical studies have been complemented by a number of recent prospective cohort studies. In a general-population birth-cohort study of 1,037 people born in Dunedin, New Zealand, and followed until age 26 years, cannabis use conferred a higher risk for the subsequent development of schizophrenia [16]. One of the strengths of this study was that it collected data on self-reported psychotic symptoms at age 11 years, to address whether psychosis preceded cannabis use. Self-reported cannabis use at both ages 15 and 18 years was also measured. Further, the entire sample was assessed at age 26 years using a standardized psychiatric interview that allowed the determination of both schizophrenia symptoms and categorical disorder. Compared to non-users, individuals using cannabis at ages 15 and 18 years had higher rates of psychotic symptoms and schizophreniform disorder at age 26 years, even after controlling for psychotic symptoms predating the onset of cannabis use. Cannabis users at age 15 years had a higher rate (OD = 3.1) of developing schizophreniform disorder at age 26 years, even after controlling for psychotic symptoms predating the onset of cannabis use.
