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. Still, the research suggests that cannabis users should think twice before using the strong stuff, especially if they use the drug on a regular basis. So the more people who used the drug daily; and the more who used high-potency marijuana, the higher the rate of psychosis.
Overall, daily cannabis users were three times more likely to suffer psychosis than those who did not use the drug, and those that used high-potency cannabis daily were five times more likely to experience a psychotic episode.
Psychotic disorders - in which people lose touch with reality - are typically triggered by factors including genetics and the environment.
In Amsterdam and London - where high-THC marijuana has always been the rule rather than the exception - 50 and 30 per cent of new psychosis cases, respectively, were associated with potent forms of the drug.
The study analyzed information from more than 1,200 people without psychosis living in 10 European cities and one city in Brazil, and compared them with 900 people living in those same cities who were diagnosed for the first time with psychosis.
The number of people having first-time psychotic episodes in Amsterdam could be halved if strong marijuana was no longer available, according to a report by King's College London researchers.
In total 29.5% of the patients had a daily cannabis habit compared with 6.8% of the "controls" untroubled by serious mental health problems.
The study was published yesterday (March 19) in the journal The Lancet Psychiatry.
Those who used ultra-potent cannabis daily were five times more likely to have a first psychotic episode - leading researchers to believe that THC may be the last straw for otherwise mentally healthy individuals.
High-strength cannabis, such as skunk, has levels of the psychoactive substance tetrahydrocannabinol (THC) above 10%.
"If you use cannabis, it doesn't mean you are definitely going to develop psychosis".
Still, the new study cannot rule out "reverse causation", meaning it could be that people with psychosis are more likely to use marijuana than people without the mental health condition, according to Suzanne Gage, of the University of Liverpool's Department of Psychological Sciences, who wrote a commentary accompanying the article. For those who used high-potency marijuana daily, the risk jumped to almost five times.
Dr Adrian James, from the Royal College of Psychiatrists, said: "This is a good quality study and the results need to be taken seriously".
David Nutt, head of the centre for neuropsychopharmacology at Imperial College London, said it was "important to realise that THC is well known to produce psychosis in healthy volunteers - people without a predisposition to mental illness".