Smoking potent 'skunk-like' cannabis increases your risk of serious mental illness, say researchers.
They estimate around one in 10 new cases of psychosis may be associated with strong cannabis, based on their study of European cities and towns.
In London and Amsterdam, where most of the cannabis that is sold is very strong, the risk could be much more, they say in The Lancet Psychiatry.
Daily use of any cannabis also makes psychosis more likely, they found.
Experts say people should be aware of the potential risks to health, even though the study is not definitive proof of harm.
Lead researcher and psychiatrist Dr Marta Di Forti said: "If you decide to use high potency cannabis bear in mind there is this potential risk."
Dr Adrian James from the Royal College of Psychiatrists said: "This is a good quality study and the results need to be taken seriously."
People experiencing psychosis lose touch with reality, and may hear voices, see things that are not actually there or have delusional, confused thoughts. It is a recognised medical condition and different to getting high on a drug.
There is disagreement as to what extent cannabis might cause or worsen mental health problems and many countries have gone ahead and legalised or decriminalised cannabis use.
Doctors are concerned about the growing use of high potency cannabis that contains lots of the ingredient THC – the one that gives the high.
Skunk-like cannabis with a THC content of 14% now makes up 94% of the drug sold on the streets of London, according to experts.
'It plagued my life'
Ad Gridley, who currently takes three different anti-psychotics, has suffered with schizophrenia and has tried to take his own life. He believes his psychosis is down to cannabis use. He no longer smokes.
"I was smoking so much it was common place for me to be stoned, and I started doing it a lot by myself too. After a couple of suicide attempts – that I didn't really admit to – my mum saw me at home in my flat hugging my knees, rocking, and she knew immediately something was wrong," he told the Victoria Derbyshire programme.
"Within 24 hours there was a GP out, and I was in hospital the next day. It plagued my life for about ten years after that. I couldn't function, and I was in and out of hospital ten times. I wasn't doing anything meaningful with my life.
"When I stopped, the psychosis stopped. I was on medications and things but that was to rectify what happened before, so my brain chemistry could get some sort of equilibrium. When I stopped smoking, the symptoms disappeared.
"If I had known the risks I doubt I would have taken it."
The researchers, from King's College London, looked at cannabis use by people in 11 EU towns and cities, including London, as well as one region of Brazil.
They compared a sample of 901 people who had experienced psychosis with 1,237 (from the general population) who had not.
They categorised the type of cannabis used by the participants according to strength, although they did not do any lab tests to measure the strength directly.
Low potency cannabis was any illicit product thought to contain less than 10% concentration of the ingredient THC. High potency was anything containing more than 10% THC.
The researchers found:
- Self-reported daily cannabis use was more common among patients with first episode psychosis, compared to controls – 29.5% (or 266 out of 901) of patients versus 6.8% (84/1,237) of controls
- High-potency cannabis use was also more common among patients with first episode psychosis, compared to controls – 37.1% (334/901) versus 19.4% (240/1,237)
- Across the 11 sites, people who used cannabis on a daily basis were three times more likely to have a diagnosis of first episode psychosis, compared with people who had never used cannabis
- This increased to five times more likely for daily use of high potency cannabis
- There was no evidence of an association between less than-weekly cannabis use and psychosis, regardless of potency
The authors estimate that one in five new cases (20.4%) of psychosis across the 11 sites may be linked to daily cannabis use, and one in ten (12.2%) linked to use of high potency cannabis.
In London, a fifth (21%) of new cases of psychosis might be linked to daily cannabis use, and nearly a third (30%) to high potency cannabis.
Removing strong cannabis from the market would lower London's psychosis incidence rate from 45.7 to 31.9 cases per 100,000 people per year, the scientists estimate.
For the South London region they looked at, that would mean 60 fewer cases of psychosis each year.
Is mild pot and occasional use OK?
Nick Hickmott from the drug and alcohol charity Addaction said: "We've got a problem with potency. People who regularly take lots of high strength cannabis are at risk of potentially serious harm. It can be particularly harmful for younger, developing brains.
"My advice is avoid using high-strength cannabis every day and pay attention to how it makes you feel. If you end up feeling anxious or just unsettled then it might be best to give it a miss. It's also not a good idea to mix it with alcohol or other drugs.
"It's also important not to over-react. Lots of people experiment with cannabis and then move on without any problems. For people who do need advice or help I'd recommend reaching out to a GP or a local drug service."
Cannabis can vary in strength and type. Skunk tends to contain higher levels of THC than weed.