Cannabis use and risk of psychotic or affective mental health outcomes

Further evidence that using cannabis could increase the risk of developing a psychotic illness such as schizophrenia, later in life, is revealed today in a study published in The Lancet.

Cannabis, or marijuana, is the most commonly used illegal substance in most countries, including the UK and USA. About 20 per cent of young people now report using cannabis at least once per week.

Dr Stanley Zammit from Bristol and Cardiff Universities, and colleagues at the universities of Bristol, Cambridge and Imperial College London, analysed 35 studies dated up to 2006. They assessed the strength of evidence for a causal relationship between cannabis use and the occurrence of psychotic or other mental health disorders.

The study, funded by the Department of Health and based in the University of Bristol, found that individuals who used cannabis were 41 per cent more likely to have any psychosis than those who had never used the drug. The risk increased relative to dose, with the most frequent cannabis users more than twice as likely to have a psychotic outcome.

Professor Glyn Lewis from the University of Bristol, and senior author on the paper, said: "It is difficult to be certain about whether cannabis use causes psychotic illnesses such as schizophrenia. It is possible that the people who use cannabis might have other characteristics that themselves increase risk of psychotic illness . However, all the studies have found an association and it seems appropriate to warn members of the public about the possible risk."

Dr Zammit  added: "Policymakers want to provide the public with advice about this widely used drug. However, even if cannabis does cause an increase in risk of developing psychosis most people who use cannabis will not develop such an illness. Nevertheless, we would still advise people to avoid or limit their use of this drug, especially if they start to develop any mental health symptoms or if they have relatives with psychotic illnesses."

At present, there is little good evidence suggesting that cannabis use increases the risk of depression, suicidal thoughts, and anxiety. Further research could help to decide whether there is a link.

The authors estimate that if cannabis had a causal relationship with psychosis, about 14 per cent of psychotic illnesses in young adults in the UK could be prevented if cannabis were not consumed.

Paper : Cannabis use and risk of psychotic or affective mental health outcomes: a systematic review by Theresa H M Moore (University of Bristol ), Stanley Zammit (Cardiff University & University of Bristol), Anne Lingford-Hughes (University of Bristol), Thomas R E Barnes (Imperial College London), Peter B Jones (University of Cambridge), Margaret Burke (University of Bristol), Glyn Lewis (University of Bristol), is published in The Lancet (July 27).