Cannabis use and psychosis: the origins and implications of an association

Abstract

Evidence for the effectiveness of treatment or secondary prevention of psychotic illness such as schizophrenia is often disappointing. This situation reflects our limited understanding of the aetiology of psychosis. There is good evidence that both genetic and environmental factors are implicated but the precise identity of these is unclear. Cannabis use is one candidate as a possible, modifiable environmental influence on both incidence and prognosis of psychosis. Evidence supporting this candidature is exclusively observational, and its strength has perhaps been overestimated and problems related to its interpretation underestimated by some. Nevertheless the possibility that cannabis does cause psychosis remains. Because of this, and because there are other good public health reasons to prevent cannabis use, interventions targeting use need to be evaluated. This evaluation, along with other imaginative approaches to future research, is needed to further our understanding of the determinants of mental illness and how we can most effectively improve the population's mental health.John Macleod's clinical interest in problem drug use dates from his work in Edinburgh in the 1990s as a general practitioner and as medical officer for a street sex-workers' outreach project. Supported by the Wellcome Trust, he trained in epidemiology at the London School of Hygiene and Tropical Medicine, after which he worked as a GP in Birmingham, where he undertook epidemiological research at the University. He is now Reader in Clinical Epidemiology and Primary Care at the University of Bristol (Department of Social Medicine, University of Bristol, Canynge Hall, Whiteladies Road, Bristol BS8 2PR, UK. Email: john.macleod@bristol.ac.uk). He is involved in ongoing studies of the causes and consequences of drug use based in Edinburgh and Bristol.

The Epidemiology of Addiction in Sub-Saharan Africa: A Synthesis of Reports, Reviews, and Original Articles

“…2 Although the association of cannabis use and severe mental illness has been reported in Africa for years, it is only recently that this association has been established internationally. 63,64 Cannabis is the third most frequently used substance by youths in Africa and up to 12-40% of young people admitted to psychiatric hospitals in Africa are diagnosed with cannabis or drug-induced psychosis. 8,65 Interventions with young people to reduce substance use could lessen the costs of mental health treatment and protect the mental health of thousands of young people in Africa.…”

Cannabis use by people with severe mental illness – is it important?

“…Macleod, however, writing in Advances in Psychiatric Treatment, critically examined the scientific evidence of a causal association between cannabis use and psychosis and concluded that, at present, evidence for this association is not strong. Nevertheless, he calls for better observational and genetic studies and for interventions targeting cannabis use, especially in the young (Macleod, 2007).…”

What every adolescent needs to know: Cannabis can cause psychosis

“…This logic is spurious as cannabis is clearly not the only factor that causes psychosis and the impact of cannabis will be muted if the potency of some of the other factors has declined. Others who are skeptical about the evidence that cannabis is a cause of schizophrenia argue that the relationship is modest in size and can be explained by confounding variables and noncausal mechanisms [51,52].…”