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. What's more, those who reported using high-potency marijuana on a daily basis were five times more likely to have a diagnosis of first-episode psychosis, compared with those who never used it.
Symptoms of psychosis include hallucination and delusion that have lasted at least for a week.
Nearly all the cannabis available in south London is a potent form known as skunk, which is estimated to be responsible for 60 new cases of psychosis a year among a population of half a million people.
High-strength cannabis, such as skunk, has levels of the psychoactive substance tetrahydrocannabinol (THC) above 10%. It was paid for by funders including Britain's Medical Research Council, the Sao Paulo Research Foundation and the Wellcome Trust.
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.
The link between marajuana use and psychosis was particularly noticeable among patients in Amsterdam and London, where high-potency cannabis is very common: In Amsterdam, 4 in 10 new cases of psychosis were linked to daily cannabis use, while half of all cases were linked to high-potency use.
These were then compared to 1,237 healthy adults to tease out the risk factors associated with psychosis. If you use cannabis, it doesn't mean you are definitely going to develop psychosis.
Dr Adrian James, the registrar of the Royal College of Psychiatrists, said: "A good drugs strategy should focus on preventing and reducing harm, not on diverting people to the criminal justice system", adding that well-provisioned and staffed addiction services needed to be restored.
It's also possible that the people who experience psychotic episodes may be more likely to use marijuana, Gage points out in an accompanying editorial to the study.
As New York and many other states are heading down the path toward marijuana legalization, public health officials, scientists and parents alike are pushing for expedited research on its dangers.
Health Canada warns that cannabis use increases the risk of developing mental illnesses such as psychosis or schizophrenia, especially for people who start young, use frequently, or have a family history of mental illness.