British study links high potency cannabis to psychosis

A study published on Tuesday in British medical journal, the Lancet Psychiatry, states that high potency cannabis or heavy use of cannabis can induce psychosis and can be harmful for mental health.

Dr. Marta Di Forti, lead author of study and clinician scientist at the Institute of Psychiatry, Psychology & Neuroscience, King’s College London, said that psychotic disorder was specifically studied.

The researchers examined data from 11 different sites that treat psychosis – 10 across Europe and one in Brazil. They looked at 901 patients who had a first-time episode of psychosis over five years and compared them to more than 1,237 matched non-patients.  

“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,” they wrote. “This increased to five times more likely for daily use of high potency cannabis.”

The researchers estimated that overall, those who consume cannabis daily are three times more likely to have a first episode psychosis than those who have never used cannabis. If the cannabis is high potency, the risk increased to five times more likely.

Dr. Michael Bloomfield of the Translational Psychiatry Research Group, was not a part of the study, but said that the findings are important and that the research “adds weight to the advice that people who use cannabis recreationally should avoid high-THC cannabis.”

“Cannabis carries severe health risks and users have a higher chance of developing psychosis,” he told Science Media Centre. “The risks are increased when the drug is high in potency, used by children and young people and when taken frequently.”

Professor of Psychiatry & Cognitive Neuroscience at King’s College London, Dr. Philip McGuire, told sources that the findings are not necessarily new information. Even so, he said that the study “involves relatively large numbers of subjects and has controlled for other risk factors that might have accounted for the results”.

According to McGuire’s own research “if healthy volunteers are given THC this induces transient psychotic symptoms like paranoia. However, if volunteers are given CBD beforehand, this blocks the induction of psychotic symptoms by THC”.

“The net effect of cannabis that contains both THC and CBD depends on the relative amounts of each,” he said. “The cannabis that was available in the 1960s was relatively low in THC and high in CBD. However, these days illicit cannabis is often ‘high potency,’ with a high THC content and a low CBD content.”

“We are currently conducting research to define the ratio of CBD:THC in cannabis that is optimal for minimizing its psychotic effects,” he said.

In response to the study, University of Liverpool Suzanne Gage has said that there are other ways of interpreting the data.

“While cannabis use has increased in some populations, the corresponding level of psychosis incidence has not,” wrote Gage.

She said that there are some people who have a genetic predisposition to both cannabis and schizophrenia. According to Gage, it’s possible that people who have psychosis may be more disposed to using cannabis heavily and that it could also be “bidirectional”, meaning it works different ways for different people.

“The next priority is to identify which individuals are at risk from daily potent cannabis use,” said Gage.