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Link between cannabis and psychosis gets more complicated

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The causal relationship between cannabis use and schizophrenia has become murkier.

A study of the genetic profile of 2082 healthy Australian adults has found that genes known to be associated with schizophrenia were present in the half who had used cannabis, raising questions about the direction of causality.

The researchers, from King’s College London and the QIMR Berghofer Medical Research Institute, suggested that, rather than the use of cannabis being a cause of mental illness, it might be that the same gene was responsible for both cannabis use and schizophrenia.

“This is an important subtlety to consider when calculating the economic and health impact of cannabis use,” the researchers said, noting that there might be a causal relationship in both directions – a possibility that required more research.

Dr Matthew Large, from Sydney University’s School of Psychiatry, told The Australian there was plenty of evidence that using cannabis precipitated psychosis, and probably caused it in many cases.

But he admitted that this had never explained “why more than half of all people with psychosis smoke cannabis”.

“The presence of a shared genetic vulnerability for psychosis could, if replicated, add greatly to our understanding of both addiction and psychosis,” Dr Large said.

The release of the research has come amid a push for the legalisation of cannabis for medicinal use.

A group of Federal parliamentarians have called for legislation to allow the sale of cannabis for medical purposes, but medical groups are cautious about the idea, and have called for more research.

Adrian Rollins