People who use higher concentrations of tetrahydrocannabinol, or THC, are more likely to have addiction and mental health problems, the study authors found.
For research, scientists have established a “standard THC unit” as 5 milligrams of THC, which produces a mild intoxication for non-regular users. Low-potency products are 5 to 10 milligrams per gram of THC, the researchers said.
“One of the highest-quality studies included in our publication found that use of high-potency cannabis, compared to low-potency cannabis, was linked to a four-fold increased risk of addiction,” Tom Freeman, PhD, the senior study author and director of the Addiction and Mental Health Group at the University of Bath in the U.K., told CNN.
Freeman and colleagues reviewed studies that looked at the links between cannabis potency and mental health and addiction. They analyzed 20 studies that included reports on anxiety, depression, psychosis, and cannabis use disorder, or marijuana addiction.
Overall, the use of higher-potency cannabis was linked to a higher risk of cannabis use disorder, as compared with use of lower-potency cannabis.
The findings appear to line up with trends in cannabis addiction and treatment rates worldwide, “while cannabis potency continued to rise during the same time,” Freeman told CNN.
During the past decade, the European Monitoring Centre for Drugs and Drug Addiction found a 76% increase in people entering treatment for cannabis addiction. In the U.S., about 3 in 10 people who use marijuana have cannabis use disorder, according to the CDC.
What’s more, a report by the United Nations found that in the past 2 decades, the “proportion of people seeking treatment for cannabis addiction has risen in all world regions apart from Africa,” Freeman said.
In a gram of herbal cannabis, THC concentrations have increased by about 2.9 milligrams each year, according to another study by Freeman and colleagues, which was published in 2020. In the cannabis resin used to make extracts and concentrations, THC levels increased about 5.7 milligrams each year between 1975 to 2017, they found.
Consumers may not know about the potency of their product or what the potency means, Freeman said. Those who buy from a store where marijuana is legally sold may be able to review a product label, but those who buy cannabis illegally “may not be able to access reliable information about the potency of the product they are using,” he said.
Although people may try to adjust the amount that they consume by “adding less cannabis to their joint or inhaling less deeply,” Freeman said, it may not work as well as they intended. High-potency products still deliver a larger dose of THC than low-potency products, he noted.
In the review, Freeman and colleagues also found that more potent cannabis was linked to more cases of marijuana-associated psychosis. This could mean a “loss of contact with reality,” including delusions and hearing voices, he told CNN.
But the association with anxiety and depression was varied across the studies, “meaning that the impact is unclear for these other mental health outcomes,” Freeman said.