Cannabis and psychosis

doi:10.1136/bmj.332.7534.172

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David M Fergusson (david.fergusson@chmeds.ac.nz), professor,
Richie Poulton, associate professor,
Paul F Smith, professor,
Joseph M Boden, research fellow

Christchurch School of Medicine and Health Sciences, University of Otago, PO Box 4345, Christchurch, New Zealand
Dunedin School of Medicine, University of Otago
Otago School of Medical Sciences, University of Otago

Correspondence to: D M Fergusson

Accepted 23 November 2005

The UK government is considering reclassifying cannabis because of concerns about links with mental health problems. What does the evidence show?

The link between cannabis and psychosis has been extensively investigated in both epidemiological and neuroscientific studies. Epidemiological studies focus on the association between use of cannabis and development of psychosis (box), whereas neuroscientific studies have looked at how cannabis affects neurochemical functioning. However, these two lines of research have been poorly integrated, with little disciplinary cross fertilisation. We have brought together both strands of evidence to give a broader picture.

Epidemiological evidence

Contemporary interest in this topic began with a longitudinal study of Swedish conscripts reported by Andreasson and his colleagues.1 Their findings have been replicated and extended in a series of longitudinal studies2^–6 all of which have found increased rates of psychosis or psychotic symptoms in people using cannabis (table). Furthermore, these findings of longitudinal, case-control studies have been augmented by a series of cross-sectional studies of large populations7 and high risk populations.8^–11 These studies produce the following suggestive evidence that supports the conclusion that the link between the use of cannabis and increased risks of psychosis is likely to be causal.

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Summary of prospective studies of cannabis use and psychotic symptoms

Demonstrator for the legalisation of cannabis

Credit: PHILLIPPE HAYS/REX

What is psychosis?

Psychosis is used in this research as a generic description of severe mental illness characterised by the presence of delusions, hallucinations, and other associated cognitive and behavioural impairments that interfere with the ability to meet the ordinary demands of life.

It is measured either by using standardised diagnostic criteria for psychotic conditions such as schizophrenia or by using validated scales that rank the level of psychotic symptoms from none to severe.

Association—All studies found that the use of cannabis is …