High-THC cannabis use strongly linked to psychotic disorders such as schizophrenia and paranoia, study finds

High potency cannabis, especially when used daily, is “strongly linked” to the risk of developing psychotic disorders such as schizophrenia and paranoia, scientists said.

In Amsterdam and London – where high-THC marijuana has long been the rule rather than the exception – 50 and 30 per cent of new psychosis cases, respectively, were associated with potent forms of the drug.

The findings, reported in medical journal The Lancet, bolster a growing body of research connecting pot use to a range of mental health disorders. With piecemeal legalisation and decriminalisation, consumption in North America and Europe has increased markedly over the last two decades, even as levels of the drug’s mind-bending molecule, known as tetrahydrocannabinol (THC), have risen four- or five-fold.

“The use of cannabis with a high concentration of THC has more harmful effects on mental health than the use of weaker forms,” said lead author Marta Di Forti, a professor at the Institute of Psychiatry, Psychology and Neuroscience at King’s College London. “Our findings also indicate for the first time how cannabis use affects the incidence of psychotic disorder at a population level.”

In 11 European cities or regions, one in five new cases of psychosis were associated with daily cannabis use, and one in 10 to high potency pot, the study found.

If high-THC cannabis – including strains such as “trainwreck”, “gorilla glue”, and “Hindu kush” – were no longer available, “12 per cent of cases of first-episode psychosis could be prevented across Europe”, the researchers calculated. In Amsterdam, the incidence of first-episode psychosis – the first time a person experiences psychotic symptoms – would likely drop from 38 to 19 per 100,000 people per year, while in London, the number of cases would decline from 46 to 32 per 100,000 people.