But experts say the new study's findings have implications for jurisdictions legalizing marijuana, warning they should consider the potential impact on their mental health services.
Smoking high-potency marijuana every day could increase the chances of developing psychosis by almost five times, according to the biggest-ever study to examine the impact of pot on psychotic disorder rates.
The team involved 901 individuals who had experienced first-time psychosis and 1,237 healthy controls in their study. "If you use cannabis, it doesn't mean you are definitely going to develop psychosis". For those who used high-potency marijuana daily, the risk jumped to almost five times.
Symptoms of this mental health disorder include psychosis or having delusions and hallucinations.
"Our findings are consistent with previous studies showing that the use of cannabis with a high concentration of THC has more harmful effects on mental health than the use of weaker forms", said Marta Di Forti, lead author from the Institute of Psychiatry, Psychology and Neuroscience at King's College London. If high-potency cannabis were no longer available, 12.2 percent of cases of first-episode psychosis could be prevented across the 11 sites. The study covered the United Kingdom, the Netherlands, France, Spain, Italy, and Brazil.
Participants were made to answer a questionnaire to know if they used cannabis in their lifetime; if the answer was affirmative, they were requested to give details on their pattern of use. "Di Forti said, "...twenty years ago, there wasn't much high potency cannabis available [in the market]". The high potency cannabis is called skunk in the UK.
Overall, cannabis use was found to be more common among patients experiencing psychosis.
The authors noted that certain regions in London, Paris and Amsterdam have a higher incidence of new cases of psychosis. The population-attributable fractions for the patterns of cannabis use associated with the highest odds of psychosis were examined, assuming causality, and the correlation between these patterns and incidence rates for psychotic disorders were assessed across study sites.
So how common is high-potency weed, anyway?
"If we think there's something particular about (high-potency) cannabis, then making that harder to get a hold of, could be a useful harm-reduction measure", said Suzanne Gage, of the University of Liverpool, who was not connected to the new study. The scientists also classified the potency of the cannabis consumed as either "high" (over 10 percent THC) or "low" (under 10 percent THC).