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

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Zammit, Stanley, Allebeck, Peter, Andreasson, Sven, Lundberg, Ingvar, Lewis, Glyn·2002·BMJ
DOI 10.1136/bmj.325.7374.1199PMC PMC135490
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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: 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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Crossref · 10.1136/bmj.325.7374.1199

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  • In the Swedish Conscript Study, those who reported using cannabis more than 50 times by age 18 had a 3-fold increase in risk of incident schizophrenia by age 45 (95% CI 1.7, 5.5).

    “The Swedish Conscript Study found a dose-response relationship between cannabis use by age 18 and incident schizophrenia by age 45 (11,12), with a 3-fold increase in risk in those who reported using cannabis more than 50 times by age 18 (95% CI 1.7, 5.5).”

    From Association Between Cannabis and Psychosis: Epidemiologic Evidence (2016) · cites this paper as [12]

    The Ever used cannabis section reports a dose-dependent association and an adjusted odds ratio of 3.1 (95% CI 1.7 to 5.5) for use on more than 50 occasions. The age limits are less precise: the Subjects section says conscripts were predominantly aged 18–20, not all 18, and the Follow up section says outcomes were tracked through 1996 rather than establishing schizophrenia by age 45 for everyone. (LLM, conf=0.86)

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  • The Swedish Conscript Study found a dose-response relationship between cannabis use by age 18 and incident schizophrenia by age 45.

    “The Swedish Conscript Study found a dose-response relationship between cannabis use by age 18 and incident schizophrenia by age 45 (11,12), with a 3-fold increase in risk in those who reported using cannabis more than 50 times by age 18 (95% CI 1.7, 5.5).”

    From Association Between Cannabis and Psychosis: Epidemiologic Evidence (2016) · cites this paper as [12]

    The Ever used cannabis section reports a dose-dependent association and an adjusted odds ratio of 3.1 (95% CI 1.7–5.5) for use on more than 50 occasions. The age cutoffs are less precise than the claim: Subjects says most men were 18–20 at conscription, when cannabis use was recorded, and Follow up says schizophrenia admissions were tracked through 1996—approximately 26–27 years later, not uniformly by age 45. (LLM, conf=0.85)

    Experimental — accuracy not yet measured

  • Studies that assessed hospitalisation for schizophrenia are unlikely to be substantially affected by bias from intoxication-related misclassification.

    “Studies that assessed hospitalisation for schizophrenia are unlikely to be substantially affected by this bias (12,16).”

    From Association Between Cannabis and Psychosis: Epidemiologic Evidence (2016) · cites this paper as [12]

    The Follow up section says cannabis-induced toxic psychoses or acute drug-induced psychoses were unlikely to have been diagnosed as schizophrenia in this Swedish cohort. It attributes that protection to Sweden’s restrictive diagnostic tradition, so the passage does not establish that hospitalisation-based studies generally are unlikely to be substantially affected by this bias. (LLM, conf=0.83)

    Experimental — accuracy not yet measured

  • In the Swedish cohort, cumulative cannabis use was associated with schizophrenia diagnosis with a linear trend OR 1.2 (95% CI 1.1, 1.4).

    “| Swedish | 50087 | Cumulative | Schizophrenia | Linear | Large sample | Only males |”

    From Association Between Cannabis and Psychosis: Epidemiologic Evidence (2016) · cites this paper as [12]

  • A number of studies found evidence for specificity of exposure, namely that associations between other drug use and psychosis are weaker than for cannabis.

    “For example, a number of studies (12)(13)(14), although not all (16,23) found evidence for specificity of exposure, namely that associations between other drug use and psychosis are weaker than for cannabis.”

    From Association Between Cannabis and Psychosis: Epidemiologic Evidence (2016) · cites this paper as [12]

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Research works printed but not linked: 1

  1. Kristjansson E, Allebeck P, Wistedt B. Validity of the diagnosis of schizo- phrenia in a psychiatric inpatient register. Nordisk Psychiatrisk Tidskrift 1987;41:229-34.

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Authors
Zammit, S., Allebeck, P., Andreasson, S., Lundberg, I., & Lewis, G.
Affiliations
  • University of Wales (GB)
  • Karolinska Institutet (SE)
  • University of Bristol (GB)
Corresponding author
Zammit, Stanley
ORCID
Venue
BMJ
Publisher
BMJ
Published
2002-11-23
Volume
325
Issue
7374
Pages
1199
ISSN
  • 0007-1447 (ISSN-L)
  • 0959-8138
  • 1468-5833
DOI
10.1136/bmj.325.7374.1199
PMC
PMC135490
OpenAlex
W2110339257
Cited by
871
References
28
Citations by year
2026: 8 · 2025: 13 · 2024: 13 · 2023: 18 · 2022: 26 · 2021: 26 · 2020: 33 · 2019: 24 · 2018: 30 · 2017: 33 · 2016: 38 · 2015: 35 · 2014: 44 · 2013: 42 · 2012: 46
FWCI
30.30
Type
journal article
Language
en
Open access
Licence
www.bmj.com/company/legal-information/terms-conditions/legal-information/tdm-licencepolicy (tdm)
Locations
  • BMJ (publishedVersion)
  • Europe PMC (PubMed Central) (publishedVersion)
  • PubMed Central (publishedVersion)
  • PubMed (publishedVersion)
  • Explore Bristol Research (publishedVersion)
  • ORCA Online Research @Cardiff (Cardiff University) (acceptedVersion)
  • UCL Discovery (University College London) (submittedVersion)
Topics
  • Health Sciences › Medicine › Pharmacology › Cannabis and Cannabinoid Research
  • Health Sciences › Medicine › Psychiatry and Mental health › Schizophrenia research and treatment
  • Health Sciences › Medicine › Epidemiology › Substance Abuse Treatment and Outcomes
Keywords
cannabis use, schizophrenia, psychosis, adolescent drug use, dose-response relationship, self-medication, amphetamine use, personality traits, sociability, conscripts, hospitalization, causal relationship
MeSH terms
Adult, Marijuana Abuse, Humans, Male, Risk Factors, Schizophrenia, Sweden, Cohort Studies, Logistic Models, Odds Ratio

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