How Much Cannabis Is Too Much? New Study Suggests Weekly THC Limits

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A new study published in Addiction suggests that weekly THC intake may help predict the risk of developing cannabis use disorder (CUD), with lower thresholds for teens than adults.

As cannabis use becomes more common, especially among teens and young adults, researchers are trying to better understand how much use may increase the risk of developing CUD — a condition marked by difficulty controlling use, cravings and continued use despite negative effects on health, school, work or relationships. Approximately 22% of people who use cannabis develop CUD.

The findings of the research could help shape safer cannabis use guidelines, similar to alcohol consumption limits.

“The ultimate goal of our new guidelines is to reduce harm,” said lead investigator Rachel Lees Thorne, Ph.D., a researcher in the Addiction and Mental Health Group, Department of Psychology, at the University of Bath, U.K. “The only truly safe level of cannabis use is no use. However, for those who don’t want to stop or are unable to, we still want to make it easier for them to lower their risk of harm. For instance, a person might opt to use lower-THC products or reduce the quantity of cannabis they use. These guidelines are designed to offer realistic, evidence-based advice for those who want to make informed choices.”

In this study, the team of investigators followed 150 people in the U.K. over one year, including adolescents ages 16 to 17 and adults ages 26 to 29. Everyone in the study used cannabis at least once during the year. The goal was to find out whether there are weekly risk thresholds of cannabis use that make CUD more likely, and whether those thresholds differ for teens and adults.

To do this, investigators measured cannabis use using standard THC units, where one unit equals 5 milligrams of THC, the main psychoactive ingredient in cannabis. This approach helped account for differences in how strong cannabis products can be. Participants reported how often they used cannabis, how much they used and the potency of the products, every three months.

At the end of the year, participants were evaluated for CUD using standard diagnostic criteria.

What did the investigators find?

• The amount of THC someone used per week was strongly linked to their risk of having CUD.
• Adolescents reached higher risk levels at much lower amounts of THC than adults.
• For adults, using about eight standard THC units per week (roughly 40 milligrams THC) was linked to a higher risk of developing any level of CUD.
• For adolescents, risk increased at around six standard THC units per week (about 30 milligrams THC).
• When looking at moderate to severe CUD, the risk threshold was higher for adults (about 13 units per week) but stayed low for adolescents (just over six units per week).

Why does this matter?

CUD doesn’t happen overnight, and many people assume that frequent cannabis use is low risk, especially if it’s legal or socially accepted. This study suggests that there may be measurable weekly limits that could help people reduce their risk, similar to guidelines used for alcohol.

The findings also highlight that teens and young people may be more vulnerable to the effects of THC, meaning smaller amounts could carry greater risks.

“Safer use thresholds based on standard THC units could help people better understand their level of use and make informed choices about their health,” said co-investigator Tom P. Freeman, B.Sc., Ph.D., a professor of psychology and the director of the Addiction and Mental Health Group at the University of Bath. “Such thresholds could be used by public health bodies and in health care settings for communicating the risks of an individual’s level of consumption, and for tracking reductions in use. As cannabis becomes increasingly available in legal markets around the world, it is more important than ever to help consumers make informed choices about their use.”