Researchers say they have moved closer to pinning down the genes which make cannabis users vulnerable to developing serious mental illness.
The British research team has found that daily cannabis users face a seven times increased risk of developing a psychotic disorder – if they have a particular variant of the AKT1 gene.
Occasional users with the gene variant faced a doubled risk.
The researchers at King’s College, London, UK, compared 278 healthy people with some 489 patients suffering their first episode of psychosis.
Although the numbers in the study were small, the researchers say their work confirms other similar findings.
The researchers say the findings could be important if medical use of cannabis is developed.
The research has been published in the journal <i>Biological Psychiatry</i> today.
Researcher Dr Marta Di Forti said: "Our findings help to explain why one cannabis user develops psychosis while his friends continue smoking without problems.
"Such findings could also help to design health educational campaigns tailored to reach those young people at particular risk."
Journal editor Dr John Krystal said the finding did not yet mean a clinically useful test.
He said: "It does show that this source of psychosis risk has a genetic underpinning."
Confirmation that the AKT1 (rs2494732) Genotype Influences the Risk of Psychosis in Cannabis Users Biological Psychiatry November 15, 2012; Volume 72, Issue 10; doi: 10.1016/j.biopsych.2012.06.020
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