The number of people having first-time psychotic episodes in Amsterdam could be halved if strong marijuana was no longer available, according to a report by King’s College London researchers.

The research, published in The Lancet Psychiatry, says banning breeds of the drug with a high THC content, would reduce the number of first-time episodes from nearly 38 per 100,000 Dutch people to nearly 19.

‘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,’ Marta Di Forti, the lead author of the research, told the Guardian.

Cannabis is classed as high strength if it has levels of the psychoactive substance tetrahydrocannabinol (THC) of above 10%.

The researchers found 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.

According to research by Dutch addiction institute Trimbos in 2017, Dutch grown marijuana sold in the city’s coffee shops has an average THC concentration of 17%. and has been going up since 2013.