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Cannabis use predicted to increase incidence of schizophrenia



Cannabis use has increased dramatically in recent years, particularly in the young. Recent studies have shown an association between cannabis use and the development of schizophrenia.

Acknowledging these recent concerns, Hickman et al have provided mathematical models designed to project trends in the incidence of schizophrenia given the increasing use of cannabis in the UK.

The authors have assumed a causal relationship between cannabis use and schizophrenia. Their models are based on different estimates of risk, including the high risk found in Swedish studies, which suggest heavy or long-term cannabis users have three times the risk of developing schizophrenia compared with non-users. The models also consider more conservative estimates of increased risk.

The modelling analysis used estimates of cannabis use derived from the Offending, Crime and Justice Survey, a large household survey conducted in 2004. The models were compared with observational data on the incidence of schizophrenia from the AESOP study, which took place between 1997 and 1999.

The paper documents a rapid increase in the prevalence of cannabis use. Between 1970 and 2002 there was a 13-fold increase in the number of people in the UK who have ever used cannabis, and an 18-fold increase in cannabis use in people under the age of 18. More than 50% of people born between 1980 and 1984 have used cannabis.

If cannabis use causes schizophrenia, then these substantial increases in use should have led to increases in the incidence and prevalence of schizophrenia. These increases have not been demonstrated, but the authors argue that recent increases in cannabis use have been most marked in the young, who have yet to experience their full lifetime risk of schizophrenia. The authors' models predict that by 2010 approximately 10-25% of new cases of schizophrenia may be caused by cannabis use.

The findings are based on assumptions about the causal relationship between cannabis use in adolescence and the development of schizophrenia. Although evidence supports an association between cannabis and schizophrenia, it is possible that the association could be explained by confounding factors, independently associated with both cannabis use and the development of schizophrenia.

More than 50% of young people between the ages of 20 and 25 will have used cannabis at some time, and many will be regular users. These patients may be putting themselves at a significantly higher risk of developing schizophrenia, and this opens up the possibility of opportunistic harm reduction advice from GPs and primary care staff.

Hickman M, Vickerman P, Macleod J, et al. Cannabis and schizophrenia: model projections of the impact of the rise in cannabis use on historical and future trends in schizophrenia in England and Wales Addiction 2007;102:597-606


Dr Jez Thompson
Former GP, Clinical Director, Leeds Community Drug Services

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