The new research adds to prior studies that have found links between marijuana and psychotic disorders, but it still does not definitively pinpoint marijuana as the cause.
At 10 locations in Europe and another in Brazil, researchers found that variation in the incidence of psychotic disorder correlated with differences in the frequency of cannabis use and, in particular, the use of high-potency cannabis, which can carry levels of psychoactive tetrahydrocannabinol (THC) greater than 10 percent. "For the primary time, now we have constant proof that there's a dose-dependent relationship between hashish use and induced psychosis at a inhabitants degree".
Writing in the journal Lancet Psychiatry, Di Forti and an worldwide 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.
"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. They used printed knowledge to estimate THC ranges within the sorts of hashish utilized by contributors. First, the researchers estimated the prevalence of psychosis by identifying all individuals with first episode psychosis who presented to mental health services between 2010 and 2015. This elevated to five instances extra possible for everyday use of excessive efficiency hashish.
Results revealed that those who regularly used cannabis were three times more likely to have psychotic episode compared to those who did not use the drug. Daily users of high potency cannabis had a four times elevated risk of psychosis, the researchers said.
"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.
Of more than 1,200 adults who participated in the study, 30 percent of the people who experienced a first psychosis event used cannabis every day. In terms of using high-THC strains, the comparison was 37 to 19 percent, respectively.
Researchers from King's College London studied 2,100 people in 11 cities in Europe and South America in the biggest study of its kind. In fact, the cities where high-potency weed is most commonly used (London, Paris, and Amsterdam) had the highest rates of new diagnoses of psychosis.
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 PAF assumes causality between daily/high potency cannabis use and first episode psychosis.