Cannabis use and schizophrenia
Is cannabis use linked to schizophrenia?
Schizophrenia and other psychotic disorders are major worldwide causes of morbidity and mortality and are a major burden on health services. Identifying risk factors is an essential part of improving global public health.
Experts agree that schizophrenia is caused by a combination of biological and environmental factors with cannabis use long suspected as a contributory factor.
Before this research was carried out cannabis was known to cause acute, short-term psychotic states but there was insufficient evidence supporting a relationship between cannabis and chronic psychotic disorders, like schizophrenia.
Examining long-term effects
In an analysis of existing research, Dr Stan Zammit and colleagues examined the complex relationship between cannabis use and its long-term effects on mental health, in particular its influence on the risk of schizophrenia.
They found that individuals who used cannabis regularly had a substantially increased risk of schizophrenia compared to those who did not use this drug.
Importantly, the data was consistent with cannabis consumption having a causal effect on psychosis which persisted even after considering the effects of other drugs and other potential risk factors.
Dr Zammit's research estimated that, if causal, approximately 15% of cases of schizophrenia could be prevented if cannabis use was eliminated in the UK.
Changing drugs policy
The research has transformed international policy and framed the debate on cannabis use. It has been widely cited in the media and used when delivering public health and educational material.
It has influenced the UK's Advisory Council on the Misuse of Drugs classification review, and was used to support statements made by the White House Office of National Drug Control Policy (ONDCP) in the US.
Dr Zammit's work was also cited by The European Monitoring Centre for Drugs and Drug Addiction and Australia's Cannabis and Mental Health National Drug Strategy. The research provides strong evidence that cannabis is one of the few modifiable risk factors in the prevention of schizophrenia.
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Professor of Psychiatric Epidemiology, Division of Psychological Medicine and Clinical Neurosciences
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- Hickman, M. et al., 2009. If cannabis caused schizophrenia - how many cannabis users may need to be prevented in order to prevent one case of schizophrenia? England and Wales calculations. Addiction 104 (11), pp.1856-1861. (10.1111/j.1360-0443.2009.02736.x)
- Moore, T. H. M. et al., 2007. Cannabis use and risk of developing psychotic or affective mental health outcomes: a systematic review. The Lancet 370 (9584), pp.319-328. (10.1016/S0140-6736(07)61162-3)
- Zammit, S. 2004. An investigation into the use of cannabis and tobacco as risk factors for schizophrenia. PhD Thesis , Cardiff University.
- Zammit, S. et al. 2002. Self reported cannabis use as a risk factor for schizophrenia in Swedish conscripts of 1969: historical cohort study. British Medical Journal 325 (7374), pp.1183-1184. (10.1136/bmj.325.7374.1199)