Paper

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

BMJ 2002; 325 doi: 10.1136/bmj.325.7374.1199 (Published 23 November 2002)
Cite this as: BMJ 2002;325:1199

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  1. Stanley Zammit, MRC clinical research fellow (zammits{at}cardiff.ac.uk)a,
  2. Peter Allebeck, professor of social medicineb,
  3. Sven Andreasson, associate professor of social medicinec,
  4. Ingvar Lundberg, professor of occupational epidemiologyc,
  5. Glyn Lewis, professor of psychiatric epidemiologyd
  1. a Department of Psychological Medicine, University of Wales College of Medicine, Cardiff CF14 4XN
  2. b Department of Social Medicine, Gothenburg University, Sweden
  3. c Department of Public Health Sciences, Karolinska Institute, Stockholm, Sweden
  4. dDivision of Psychiatry, University of Bristol, Bristol
  1. Correspondence to: S G Zammit
  • Accepted 12 September 2002

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, P<0.001), and for subjects who had used only cannabis and 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. Similar results were obtained when analysis was restricted to subjects developing schizophrenia after five years after conscription, to exclude prodromal cases.

Conclusions: 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

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

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

Footnotes

  • Editorial by Rey and Tennant

  • Funding This research is funded from a clinical training fellowship grant, awarded to SZ by the Medical Research Council, UK (grant no. G84/5689).

  • Competing interests None declared.

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