Date published: 2007/07/27
The BBC says:
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.
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.
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.
...
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."
In one paragraph the authors of the study admit perfectly well that they are in danger of confusing correlation and causation, but unfortunately the bias of most of the article is towards exactly pushing the idea that they have found a causation, i.e. that using cannabis is responsible for schizophrenia, and they have shown no such thing. The comment by Iverson that "the incidence of schizophrenia has not shown any significant change in the past 30 years" is one that gives a good reason to doubt the half-claim of causation by the authors of the study. As with most health studies, all we have is a correlation, not a causation, and it is unfortunate that researchers (and the BBC) insist on using misleading language to imply a causation.
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