In one of the earliest studies that associates marijuana use with schizophrenia, 45,570 Swedish conscripts were asked to report their frequency of cannabis use. Over the 15 year follow up period, conscripts who had used cannabis more than 50 times before conscription had a six times higher risk for development of schizophrenia than non-users. Those who used cannabis 11 to 50 times had a three-fold increase in risk.