A double dose of a faulty gene could be the connection between cannabis use and the development of schizophrenia, according to the New Scientist report.
In the original New Zealand study used by researchers, people who had smoked cannabis on three occasions by the age of 15 had a 10 per cent chance of developing the condition by the age of 26.
Dr Mary Cannon's research team recently re-analysed the data from from this study, adding another variable - genetic predisposition to schizophrenia.
The gene they investigated, called COMT (catechol-O-methyl transferase), encodes an enzyme that breaks down a signalling chemical dopamine in the brain.
COMT comes in two forms, one of which is marginally more common in people with schizophrenia and is thought to be a risk factor for the disease.
The results were crystal clear.
The team found that in New Zealanders with two copies of the "normal" version of COMT, smoking cannabis had little effect on their mental health. In people with one normal and one "bad" form of the gene, smoking cannabis slightly increased their risk of psychosis.
But for people with two copies of the bad gene, cannabis spelled trouble: smoking the drug as a teenager increased their likelihood of developing psychosis by a factor of 10.