Psychological Medicine

Original Articles

Cannabis, schizophrenia and other non-affective psychoses: 35 years of follow-up of a population-based cohort

E. Manrique-Garciaa1 c1, S. Zammita2, C. Dalmana3, T. Hemmingssona4, S. Andreassona5 and P. Allebecka1

a1 Department of Public Health Sciences, Division of Social Medicine, Karolinska Institutet, Stockholm, Sweden

a2 Department of Psychological Medicine and Neurology, MRC Centre for Neuropsychiatric Genetics and Genomics, Cardiff University, Cardiff, UK

a3 Department of Public Health Sciences, Division of Public Health Epidemiology, Karolinska Institutet, Stockholm, Sweden

a4 Department of Public Health Sciences, Division of Occupational and Environmental Medicine, Karolinska Institutet, Stockholm, Sweden

a5 Swedish National Institute of Public Health, Sweden

Abstract

Background There is now strong evidence that cannabis use increases the risk of psychoses including schizophrenia, but the relationship between cannabis and different psychotic disorders, as well as the mechanisms, are poorly known. We aimed to assess types of psychotic outcomes after use of cannabis in adolescence and variation in risk over time.

Method A cohort of 50 087 military conscripts with data on cannabis use in late adolescence was followed up during 35 years with regard to in-patient care for psychotic diagnoses.

Results Odds ratios for psychotic outcomes among frequent cannabis users compared with non-users were 3.7 [95% confidence interval (CI) 2.3–5.8] for schizophrenia, 2.2 (95% CI 1.0–4.7) for brief psychosis and 2.0 (95% CI 0.8–4.7) for other non-affective psychoses. Risk of schizophrenia declined over the decades in moderate users but much less so in frequent users. The presence of a brief psychosis did not increase risk of later schizophrenia more in cannabis users compared with non-users.

Conclusions Our results confirm an increased risk of schizophrenia in a long-term perspective, although the risk declined over time in moderate users.

(Received May 10 2011)

(Revised August 19 2011)

(Accepted August 30 2011)

(Online publication October 17 2011)

Correspondence

c1 Address for correspondence: E. Manrique-Garcia, M.D., Karolinska Hospital, Department of Public Health, Norrbacka floor 6, S-17176 Stockholm, Sweden. (Email: edison.manrique-garcia@ki.se)

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