Self reported cannabis use as a risk factor for schizophrenia in Swedish conscripts of 1969: historical cohort study

Abstract

Objectives: An association between use of cannabis in adolescence and subsequent risk of schizophrenia was previously reported in a follow up of Swedish conscripts. Arguments were raised that this association may be due to use of drugs other than cannabis and that personality traits may have confounded results. We performed a further analysis of this cohort to address these uncertainties while extending the follow up period to identify additional cases. Design: Historical cohort study. Setting: 1969-70 survey of Swedish conscripts (>97% of the country's male population aged 18-20). Participants: 50 087 subjects: data were available on self reported use of cannabis and other drugs, and on several social and psychological characteristics. Main outcome measures: Admissions to hospital for ICD-8/9 schizophrenia and other psychoses, as determined by record linkage. Results:Cannabis was associated with an increased risk of developing schizophrenia in a dose dependent fashion both for subjects who had ever used cannabis (adjusted odds ratio for linear trend of increasing frequency 1.2, 95% confidence interval 1.1 to 1.4, PConclusions: Cannabis use is associated with an increased risk of developing schizophrenia, consistent with a causal relation. This association is not explained by use of other psychoactive drugs or personality traits relating to social integration. What is already known about this topic Use of cannabis has been associated with an increased risk of developing schizophrenia Alternative explanations for this association include confounding by personality or by use of other drugs such as amphetamines, and use of cannabis as a form of self medication secondary to the disorder What this study adds Self reported cannabis use is associated with an increased risk of subsequently developing schizophrenia, consistent with a causal relation This association is not explained by sociability personality traits, or by use of amphetamines or other drugs Self medication with cannabis is an unlikely explanation for the association observed