Smoking potent forms of cannabis greatly increases the risk of serious mental illness, a major study has found.
Taking any version of the drug daily was found to triple the chances of developing psychosis. But using high-potency cannabis containing high levels of the active ingredient THC raised the risk five-fold.
Scientists found that the link with psychosis was strongest in London and Amsterdam, where high-potency cannabis is commonly available.
Psychotic conditions such as schizophrenia can have devastating effects, including paranoid delusions and hallucinations.
Researchers looked at the drug use history of 901 patients experiencing a first psychotic episode between 2010 and 2015, and 1,237 healthy individuals.
The study took place in England, France, the Netherlands, Italy, Spain and BraziI.
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. Just over a fifth of the psychosis cases were thought to be linked to daily cannabis use across the 11 sites.
Use of high-potency cannabis was a strong predictor of psychotic illness in London and Amsterdam, where powerful versions of the drug were widely available, said the authors.
Strong marijuana is commonly known as skunk weed.
In London, skunk-like cannabis with a THC content of 14% now makes up 94% of the drug sold on the street, according to the study.
In France and Spain, herbal forms of cannabis with a THC content of less than 10% were still commonly used.
Removing "skunk weed" from the market would drop London's psychosis incidence rate from 45.7 to 31.9 cases per 100,000 people per year, the scientists estimated.
Such a move would have an even bigger impact in Amsterdam, reducing the frequency of cases from 37.9 to 18.8 per 100,000 people per year.
The findings are published in the journal The Lancet Psychiatry.