People who drink alcohol have long had a rough guide: count your drinks, stay within a weekly limit, and you can sidestep most of the trouble. Cannabis users have had no such yardstick. Two people might both smoke a few times a week, yet one slides into dependency while the other never does. The difference, it turns out, isn’t how often you light up. It’s how much THC actually enters your body.
Researchers at the University of Bath have proposed the first evidence-based weekly limit for cannabis, built around a simple concept borrowed from alcohol guidelines: the standard unit. One THC unit equals 5 milligrams of delta-9-tetrahydrocannabinol, the compound responsible for the high. Keep your weekly total below eight units, roughly 40 milligrams, and your chances of developing cannabis use disorder drop sharply. Go above that line, and the risk starts to climb.
Why Counting Joints Never Told the Whole Story
The problem with measuring cannabis by the joint or the gram is that potency varies wildly. A single joint rolled with today’s high-strength herbal cannabis can pack nearly 13 THC units, already past the weekly threshold for safer use. Concentrates push the numbers even higher. Two people using cannabis on the same number of days might be exposing themselves to vastly different doses.
The Bath team drew their thresholds from CannTeen, a year-long study that tracked 150 cannabis users in London. They found that 80 percent of people who stayed below eight units per week did not develop a disorder. Above 13 units, the more severe forms of cannabis use disorder became increasingly common. For teenagers, the threshold was even lower, just six units.
Cannabis use disorder now affects about 22 percent of regular users. It’s defined not by how much someone uses, but by whether the drug causes significant distress or begins to interfere with work and relationships. Many who struggle with it never seek help.
“The ultimate goal of our new guidelines is to reduce harm. 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,” says Rachel Lees Thorne of the University of Bath.
A Label You Can Use
The researchers hope their work will push regulators toward better product labeling, particularly in places where cannabis is legal. Right now, the percentages printed on packages tell consumers almost nothing about their personal risk. A THC unit system could change that, giving people a number to track the way they might check units on a wine bottle.
Canada is already exploring the idea. The Bath team plans to expand their research internationally to see whether the eight-unit threshold holds across different populations and legal environments.
None of this is a guarantee. Individual risk varies, and the safest choice remains not using cannabis at all. But for the millions who do use it, the study offers something new: a way to think about the habit not as a vague behavior, but as a measurable exposure. And like counting drinks, counting units puts the decision back in your hands.
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