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

doi:10.1136/bmj.325.7374.1199

BMJ 2002;325:1199 ( 23 November )

Papers

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

Papers pp 1195, 1212

Stanley Zammit, MRC clinical research fellow, ^a Peter Allebeck, professor of social medicine, ^b Sven Andreasson, associate professor of social medicine, ^c Ingvar Lundberg, professor of occupational epidemiology, ^c Glyn Lewis, professor of psychiatric epidemiology. ^d

^a Department of Psychological Medicine, University of Wales College of Medicine, Cardiff CF14 4XN, ^b Department of Social Medicine, Gothenburg University, Sweden, ^c Department of Public Health Sciences, Karolinska Institute, Stockholm, Sweden, ^d Division of Psychiatry, University of Bristol, Bristol

Correspondence to: S G Zammit zammits{at}cardiff.ac.uk

Objectives: An association between use of cannabis inadolescence and subsequent risk of schizophrenia was previouslyreported in a follow up of Swedish conscripts. Arguments wereraised that this association may be due to use of drugs otherthan cannabis and that personality traits may have confoundedresults. We performed a further analysis of this cohort to addressthese uncertainties while extending the follow up period to identifyadditionalcases.
Design: Historical cohortstudy.
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 selfreported use of cannabis and other drugs, and on several socialand psychologicalcharacteristics.
Main outcome measures: Admissions to hospital for ICD-8/9 schizophreniaand other psychoses, as determined by recordlinkage.
Results: Cannabis was associated with an increasedrisk of developing schizophrenia in a dose dependent fashion bothfor subjects who had ever used cannabis (adjusted odds ratio forlinear trend of increasing frequency 1.2, 95% confidence interval1.1 to 1.4, P<0.001), and for subjects who had used only cannabisand no other drugs (adjusted odds ratio for linear trend 1.3,1.1 to 1.5, P<0.015). The adjusted odds ratio for using cannabis>50 times was 6.7 (2.1 to 21.7) in the cannabis only group. Similarresults were obtained when analysis was restricted to subjectsdeveloping schizophrenia after five years after conscription,to exclude prodromalcases.
Conclusions: Cannabis use is associated with an increasedrisk of developing schizophrenia, consistent with a causal relation.This association is not explained by use of other psychoactivedrugs or personality traits relating to socialintegration.

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

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