The THC concentration in cannabis has doubled in the past 10 years, raising the price and psychoactive potency, the U.K.'s Independent reported.
The report was based on a study from the University of Bath and King's College London for the European Monitoring Centre for Drugs and Drug Addiction, using data collected from 28 European Union States.
The THC concentration in herbal cannabis rose from 5 percent in 2006 to 10 percent in 2011, while THC in cannabis resin rose from 8 percent in 2006 to 10 percent in 2011 and increased to 17 percent by 2017, according to the study.
"These findings show that cannabis resin has changed rapidly across Europe, resulting in a more potent and better value product," lead author of the study Dr. Tom Freeman said, the Independent reported.
The increased potency could be more harmful, Freeman warned in his study, per the report.
"Cannabidiol (CBD) has the potential to make cannabis safer, without limiting the positive effects users seek," Freeman said, per the Independent. "What we are seeing in Europe is an increase in THC and either stable or decreasing levels of CBD, potentially making cannabis more harmful."
With more U.S. states moving to legalizing marijuana, assistant professor of psychology at Iowa State University, Dr. Brooke Arterberry told the Independent states might consider regulating the potency.
"THC has linearly increased over two decades," Arterberry told the paper. "Based on the results, states may want to think about the available potency levels of cannabis products, especially with the changing legal landscape of cannabis."