People who use cannabis every day run a significantly higher risk of developing the serious mental illness psychosis, especially if they use more potent forms of the drug, such as skunk, scientists said.
"This is more evidence that the link between cannabis and psychosis matters", says Krista M. Lisdahl, a clinical neuropsychologist at the University of Wisconsin, Milwaukee, who wasn't involved in the study.
Researchers said the study - published in The Lancet Psychiatry journal - is the first to show the impact of cannabis use on population rates of psychosis, and should highlight the potential public health impact of changes to drugs laws.
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. In London, 94 per cent of the cannabis sold on the street is skunk, which has an average THC of 14.
Researchers found that people living in south London who smoke cannabis regularly are five times as likely to develop psychosis as those who don't.
Use of high-potency weed nearly doubled the odds of having psychosis compared with someone who had never smoked weed, explains Di Forti.
Cadence of Hyrule Was Born From a "Two-Way Street" of Interest
At the Game Developers Conference event this morning, Nintendo shared which indie games have sold best on Switch up until now. A new " Cuphead " video game is coming to the Nintendo Switch , and it says a lot about the future of Xbox.
The easy availability of high-THC weed is a recent phenomenon, she notes. "Di Forti said, "...twenty years ago, there wasn't much high potency cannabis available [in the market]".
"If you are going to legalise cannabis, unless you want to pay for more a lot more psychiatric beds and a lot more psychiatrists, then you need to devise a system where you would legalise in a way that wouldn't increase the consumption and increase the potency", study author Sir Robin Murray told The Guardian.
The findings are consistent with previous experiments that suggest heavy use and high t-h-c concentration cannabis can be harmful to mental health.
"In European cities where high-potency cannabis is widely available, a significant proportion of new cases of psychosis are associated with daily cannabis use and high-potency cannabis", Di Forti said. 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.
These are also cities where high-potency weed is most easily available and commonly used.
Di Forti and her team also identified and categorized the low potency and high potency tetrahydrocannabinol or THC - the psychoactive component of cannabis.
For example, in Amsterdam, four in 10 (43.8%) new cases of psychosis were estimated to be linked to daily cannabis use, and 5 in 10 (50.3%) new cases linked to high potency use .