Writing in the journal Lancet Psychiatry, Di Forti and an worldwide team of researchers report how they studied patient data - including cannabis use - collected between mid 2010 and mid 2015 for 901 adults under the age of 65 who arrived at mental health services in one of 10 locations in Europe, or one in Brazil, and received their first diagnosis of a psychotic disorder that was not down to, for example, brain tumours or acute drug use. But using high-potency cannabis, known as "skunk", which contains high levels of the compound THC, raised the risk five-fold.
The evidence shows those types of users may actually be three times more likely to develop a psychotic disorder than someone who has never used cannabis. She added that the connection between the cities where high potency cannabis is available and incidence of psychosis has not been studied before. They they also predict that first-time psychosis cases in Amsterdam would fall from 38 to 19 per 100,000 annually, and in London from 46 to 32 per 100,000.
The researchers collected information about participants' history of cannabis use and other recreational drugs.
One recent study showed that high-potency cannabis is increasingly dominating markets.
The researchers found that compared with never users, daily cannabis use correlated with increased odds of psychotic disorder (adjusted odds ratio, 3.2); for daily use of high-potency types of cannabis, the odds were increased almost fivefold (adjusted odds ratio, 4.8).
Several past studies have found that more frequent use of pot is associated with a higher risk of psychosis - that is, when someone loses touch with reality.
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The team involved 901 individuals who had experienced first-time psychosis and 1,237 healthy controls in their study.
In some locations, THC levels are 20 per cent or higher, they noted.
"Daily use of high-potency cannabis and how this varies across Europe explains some of the striking variations we have measured in the incidence of psychotic disorder", said Di Forti.
Even the Royal College of Psychiatrists is reviewing its position to consider the view that decriminalisation would give the government power to regulate its strength and generate taxes. "You can't say that cannabis causes psychosis", she says.
Despite his skepticism, Armentano allowed that "the concerns raised in this paper ought to be taken seriously, and they provide an argument in favor of better regulation of the plant so that it can be better kept out of the hands of young people and those who may be at higher risk for an adverse reaction". It was paid for by funders including Britain's Medical Research Council, the Sao Paulo Research Foundation and the Wellcome Trust.
Experts analyzed data from 11 sites across Europe and Brazil.
For the first time it also looked at 900 psychosis patients and...