Even a few joints a week can matter ― measured in milligrams of THC. A new study led by Rachel L. Thorne at the University of Bath gives the first concrete weekly dose thresholds at which the risk of cannabis use disorder increases significantly. The study was published in the journal Addiction.
How much THC per week poses a health risk?
Researchers used data from the CannTeen study, with 85 adolescents (16–17) and 65 adults (26–29) who had used cannabis in the past year. The team defined one THC unit as 5 milligrams, analogous to the standard drink unit used in alcohol research.
Using repeated surveys of consumption over a year and a clinical diagnosis at the study’s end, the researchers derived thresholds separating typical use from cannabis use disorder. Cannabis use disorder is present when someone cannot control use and continues despite clear problems in daily life, such as neglecting school, work or family responsibilities, and experiencing withdrawal symptoms like restlessness or sleep problems when trying to quit.
For adolescents the risk threshold was about 6 THC units per week (≈30 mg THC). For adults it was about 8 units per week (≈40 mg THC). Thresholds for moderate to severe disorders were higher. The researchers noted that only abstinence is completely risk-free.
THC units based on alcohol research
Alcohol research commonly uses standard drinks or units to communicate risky consumption. Threshold values can be useful for communicating health risks, says Jakob Manthey of the Centre for Interdisciplinary Addiction Research at the University of Hamburg, though he cautions these figures can be misread as implying lower consumption is harmless.
Cannabis differs from alcohol because it contains many active ingredients whose interactions affect effects and risks. THC is the main risk factor, but other cannabinoids and the route of administration (joint, vaporizer, edible) can significantly change dose and effect.
How reliable are the THC figures?
A strength of the study is repeated measures from the same individuals across a year. Limitations include the small sample (150 people) and that THC content was estimated from external sources rather than measured in lab-analyzed samples. Thus the figures should be treated as initial guidelines, not absolute limits. Still, they show a clear pattern: higher weekly THC intake is associated with a greater risk of cannabis use disorder.
Benefits for diagnosis, therapy and prevention
The thresholds don’t replace clinicians but could aid preliminary screening. Practitioners might ask patients how many THC units they use per week to better assess risk and detect problems earlier. Treatment guidelines already emphasize frequency, quantity and potency as important risk factors.
A standardized unit system could make consumption data more comparable, but it won’t on its own change use patterns. Availability, advertising, youth protection and other measures also influence consumption.
What THC units can do ― and what they can’t
A practical issue is consumers often don’t know product THC content, especially for home-grown or illicit sources. Under current regulations there is little widespread communication of THC units, so reliable information on product potency is often unavailable, says Manthey.
British neuropsychopharmacologist David Nutt calls the analysis an important step: the data provide an estimate of a weekly consumption threshold to limit dependence risk. He advocates for a regulated cannabis market with clear product quality and unit identification, as exists for alcohol.
Proposed THC units could clarify vague terms like “a lot” or “risky.” But protecting health requires more than a weekly unit number: honest information on potency, effective prevention, and, when necessary, the willingness to limit consumption or quit.
This article was translated from German.