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2007-07-27 05:19:32
Cannabis users are 40% more likely than non-users to suffer a psychotic illness such as schizophrenia, say UK experts.
Writing in the Lancet, a team led by Dr Stanley Zammit from Bristol and Cardiff Universities said young people needed to be made aware of the dangers.
In an additional article, experts said up to 800 schizophrenia cases a year in the UK could be linked to cannabis use.
The researchers looked at 35 studies on the drug and mental health - but some experts urged caution over the results.
The study found the most frequent users of cannabis have twice the risk of non-users of developing psychotic symptoms, such as hallucinations and delusions.
But the evidence for a link with depression and anxiety was less clear, they said.
Public awareness
The authors said the risk to any individual of getting schizophrenia remained low overall, but because cannabis use was so common, they estimated it could be a factor in 14% of psychotic problems among young adults in the UK.
All the studies have found an association and it seems appropriate to warn members of the public about the possible risk
Professor Glyn Lewis
Report author
However, they said they could not rule out the possibility that people at a higher risk of mental illness were more likely to use the drug.
Study author, Professor Glyn Lewis, professor of psychiatric epidemiology, said: "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."
He added he would particularly advise users who were developing mental health problems or who had a family history of psychotic illness to quit using the drug.
In an accompanying editorial, Danish researchers said the figures presented in the research translated to about 800 potentially avoidable cases of schizophrenia a year in the UK among 15- to 34-year-olds.
Cabinet confessions
There are an estimated 2m regular users of cannabis in the UK.
Cannabis is currently a class C drug, having been downgraded from class B in 2004 - a move that made possession of the drug a largely non-arrestable offence.
Their prediction... is not supported by the fact that the incidence of schizophrenia has not shown any significant change in the past 30 years
Professor Leslie Iverson
University of Oxford
However, earlier this month, Prime Minister Gordon Brown announced a consultation on reclassifying cannabis as class B, amid reports that more potent strains such as "skunk" are becoming widely available.
In the following days a string of Cabinet ministers, including Home Secretary Jacqui Smith, Chancellor Alistair Darling and Transport Secretary Ruth Kelly, admitted having taken cannabis when they were younger.
Professor Robin Murray, professor of psychiatry at London's Institute of Psychiatry, said of the Bristol study: "The studies they looked at were done in the 70s, 80s and 90s.
"One of the questions they can't address is whether the risk is higher with the more concentrated skunk forms of cannabis, which are now widely available."
'Daily tragedy'
But Professor Leslie Iverson, from the University of Oxford, said there was still no conclusive evidence that cannabis use causes psychotic illness.
"Their prediction that 14% of psychotic outcomes in young adults in the UK may be due to cannabis use is not supported by the fact that the incidence of schizophrenia has not shown any significant change in the past 30 years."
Marjorie Wallace, chief executive of the mental health charity SANE, said: "This analysis should act as a serious warning of the dangers of regular or heavy cannabis use."
She added: "The headlines are not scaremongering but reflect a daily, and preventable, tragedy."
Professor David Nutt, head of psychopharmacology at the University of Bristol, said that cannabis was unquestionably harmful but very much less addictive or damaging than either alcohol or tobacco.
"The idea that reclassification upwards will do anything to reduce psychosis is naive and runs the risk of perversely inflicting even greater suffering - through increasing criminal sanctions - on vulnerable individuals already afflicted with mental illness."