Epidemiological studies have contributed most significantly to the evidence suggesting that cannabis can “cause” a persistent psychotic disorder. The study that first brought significant attention to the topic was a large historical, longitudinal cohort study of all Swedes conscripted between 1969 and 1970 [11]. Since Sweden mandates military service, 97% of males aged 18–20 years were included. The relationship between self-reported cannabis use at the time of conscription and psychiatric hospitalization for schizophrenia in the ensuing 15 years was examined. A dose–response relationship was observed between cannabis use at conscription (age 18 years) and schizophrenia diagnosis in the following 15 years. Individuals who reported having used cannabis more than 50 times were six times more likely than non-users to have been diagnosed with schizophrenia in the ensuing 15 years. Adjusting for other relevant risk factors reduced but did not eliminate the higher risk (odds ratio = 2.3) of schizophrenia conferred by cannabis use.
