Frequent cannabis use and high-strength varieties are likely to increase the chance of mental health problems among users, according to researchers behind the largest study of its kind.

Experts have previously flagged a link between cannabis use and psychosis, particularly among vulnerable people with heavy use of the drug. Now research suggests the potency of the cannabis is also important, with patterns in cannabis use linked to how often new cases of psychotic disorders arise in different cities.

The study estimated that 30% of first-time cases of psychotic disorders in south London, and half of those in Amsterdam, could be avoided if high-potency cannabis was not available. The team says that equates to about 60 fewer cases per year in south London.

“If you are a psychologist like me who works in this catchment area and sees first-episode psychosis patients, this has a significant impact at the level of services and, I would also argue, family and society,” said Dr Marta Di Forti, the lead author of the research, from King’s College London.

High-strength cannabis, such as skunk, has levels of the psychoactive substance tetrahydrocannabinol (THC) above 10%. According to data released last year, 94% of police cannabis seizures in the UK were of high-strength varieties. These varieties also contain very little cannabidiol (CBD), a substance that might protect against psychosis.

Writing in the journal Lancet Psychiatry, Di Forti and an international 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.

For comparison, the team asked more than 1,200 healthy individuals from across the same areas about their cannabis use. The strength of cannabis was estimated from the name individuals gave to the drug.

After taking into account factors including drinking, education and use of other drugs such as ketamine, the team found those with a psychotic disorder were more likely to have used cannabis at some point in their life than those without the condition.

Frequency of use was also highlighted by researchers: the chances of having a psychotic disorder were 40% greater among those who used the drug more than once a week compared with those who rarely, if ever, used it, while the chances of having a psychotic disorder were more than three times greater among those who used cannabis daily compared with those who rarely if ever used it.

What is more, daily users of high potency cannabis were more likely to have a psychotic disorder, compared with never-users, than those who used low-potency cannabis every day.

The biggest link between daily cannabis use and having a psychotic disorder was in Amsterdam, where the chances were seven times higher than for those who had never used the drug: almost all cannabis sold in “coffee shops” in Amsterdam is high-strength, while varieties with 67% THC have been found in the Netherlands. Incidences of psychosis were higher in Amsterdam than most other locations studied, with only south London surpassing it.

“Daily use of high-potency cannabis and how this varies across Europe explains some of the striking variations we have measured in the incidence of psychotic disorder,” said Di Forti.

However, she noted that not all daily users of high-potency cannabis develop a psychotic disorder, meaning it is important to work out who is most vulnerable, and that other factors are also at play.

The study had limitations because it relied on self-reported use of cannabis and only small numbers of participants were involved at each site. Also, THC and CBD content of the cannabis was not directly measured while the results might, at least in part, be down to those at greater risk of psychosis being more likely to use cannabis.

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.

“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,” he said.

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.