In Amsterdam and London, specifically, there was a strong tie between the use of high-potency cannabis and the presence of psychosis. The researchers linked four in 10 (43.8 percent) new cases of psychosis in Amsterdam to daily cannabis consumption and five out of 10 (50.3 percent) of new cases with the use of highly potent versions of the drug.
In London, the researchers believe they could link 21.0 percent of new cases to daily use and 30.3 percent to a preference for highly potent cannabis.
"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," notes lead study author Dr. Marta Di Forti.
"They also indicate for the first time how cannabis use affects the incidence of psychotic disorder at a population level," she adds.
"As the legal status of cannabis changes in many countries and states, and as we consider the medicinal properties of some types of cannabis, it is of vital public health importance that we also consider the potential adverse effects that are associated with daily cannabis use, especially high potency varieties."
Dr. Marta Di Forti