People who smoke cannabis on a regular basis are much more likely to experience delusions, severe depression and other symptoms of psychosis - with the risk increasing significantly with more potent forms of the drug, a new study finds.
Cannabis is classed as high strength if it has levels of the psychoactive substance tetrahydrocannabinol (THC) of above 10%.
Adults' recreational use of pot is now legal in Canada.
For comparison, the team asked more than 1,200 healthy individuals from across the same areas about their cannabis use.
It's "increasingly evident that [marijuana use is] a risk factor" for psychosis, Roane told Live Science.
Writing in the journal Lancet Psychiatry, Di Forti and an global 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.
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.
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Just over a fifth of the psychosis cases were thought to be linked to daily cannabis use across the 11 sites.
The study's findings are in line with previous research pointing to a link between cannabis use and psychosis - a condition when someone loses grip on reality - though the new research does not definitively find a causal relationship between marijuana and mental health issues.
"As the legal status of cannabis changes in many countries and states, and as we consider the medicinal properties of some types of cannabis, it is of vital public health importance to consider the potential adverse effects that are associated with daily cannabis use, especially high potency varieties", said Di Forti.
If high-potency marijuana was no longer available, researchers predicted the incidence of psychosis in Amsterdam would be slashed in half. "Cannabis for them could be an extra risk factor, but it definitely doesn't have to be involved".
Prof Sir Robin Murray, another author of the study from King's College London, said the study has implications for the debate on whether cannabis should be legalised. In Amsterdam, where cannabis has been decriminalized for decades, the frequency of first-time psychosis diagnoses would decline from 38 people per 100,000 each year to 19 people per 100,000.
Dr Adrian James, from the Royal College of Psychiatrists, said: "This is a good quality study and the results need to be taken seriously".