Cannabis users “more than twice as likely” to develop psychotic illness

Frequent cannabis users are more than twice as likely to develop a psychotic illness such as schizophrenia in later life than people who have never used the drug, according to research published today.

A report in medical journal The Lancet found that people who used cannabis were 41% more likely to have any psychosis than those who have never used the drug, and the risk increased among frequent cannabis users.

About one-fifth of young people now report using cannabis at least once a week, according to the report.

Professor Glyn Lewis from Bristol University, one of the report’s authors, said it was “difficult to be certain” whether cannabis caused 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,” he added.

The report said that if cannabis did have have a causal relationship with psychosis about 14% of psychotic illnesses in young adults in the UK could be prevented if they did not use the drug.

Earlier this week, the government published a consultation on its new drug strategy proposing a reclassification of cannabis from Class C to Class B because of concerns over “skunk” and stronger strains of the drug.

Think-tank Transform Drug Policy Foundation claimed the prime minister had included the cannabis proposal in response to the Conservatives’ recent social policy report recommending the same measure.

“The announcement was all about politics and nothing to do with science,” Transform director Danny Kushlick said.

Paul Corry, director of public affairs at mental health charity Rethink, said: “Rather than focusing its attention on the reclassification debate, the government would do well to crack on with the more important job of informing the public about the health implications.”

The Lancet study looked at 35 studies up to 2006, and was funded by the Department of Heath and based in the University of Bristol.
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