Doubling the amount of THC a person takes does not make them feel twice as high, according to a new analysis of how cannabis dose relates to intoxication. The findings were published in the journal Neuroscience & Biobehavioral Reviews and were first reported by the cannabis news site leafie.

THC (tetrahydrocannabinol) is the main psychoactive compound in cannabis. The researchers found that raising the dose generally increased feelings of intoxication, but not in proportion. Higher doses produced progressively smaller increases in the intensity of the high.

What the review found

The team combined evidence from 41 studies involving 1,020 participants who used cannabis less than once a week. Each study looked at the immediate effects of a known THC dose and compared participants’ reported experiences with those after a placebo.

After accounting for the placebo, inhaling 5mg of THC raised the reported high by about 34 points on a 100-point scale. Doubling the dose to 10mg raised the estimated effect to 39 points, an additional increase of roughly 15%.

The researchers also looked at other subjective effects. A 5mg dose increased reported anxiety by 14 points, tiredness by 26 points and calmness by five points. It reduced reported alertness by 40 points and contentment by 71 points, compared with placebo. Moving from 5mg to 10mg did not produce further changes in anxiety, tiredness or alertness in their analysis.

Writing in The Conversation, researchers Isabella Goodwin and Amir Englund said that diminishing effects at higher doses are common in pharmacology, where the effect of a substance can level off as the dose rises.

Standard THC unit and potency

The authors said the results are relevant to a proposed standard THC unit, intended to help people understand and compare cannabis consumption in the way alcohol units do. One unit would represent 5mg of THC. Their findings suggest that even a single unit can have substantial subjective effects in people who rarely use cannabis.

The research comes amid concern about rising potency in both legal and illegal markets. The authors noted, however, that THC concentration is not the same as the amount actually consumed, because habits and methods of use affect exposure. Cannabis with twice the concentration does not necessarily deliver twice the dose.

They also discussed tolerance. Regular exposure to THC can temporarily reduce the availability of cannabinoid receptors in the brain, so regular users may need larger doses to feel the effects they once had at lower doses. The researchers warned that frequent use of high-potency cannabis has been linked to a greater risk of dependence and mental health problems, particularly among vulnerable people.

Limitations

The researchers stressed that the results should not be taken to mean larger doses carry no extra risk. The study covered only infrequent users, so its conclusions may not apply to regular consumers who have built up tolerance. The analysis also pooled studies that used different forms of THC, including synthetic preparations and herbal cannabis, as well as different methods of administration and experimental settings.

The findings concern subjective intoxication rather than objective impairment. Feeling less intoxicated does not necessarily mean memory, coordination or reaction times are less affected.

The authors said the results reinforce existing harm reduction advice for inexperienced users. In their words, “the high rises quickly at low doses, but extra THC soon adds less and less, which backs the advice to start low and go slow”.

In the UK, cannabis remains a Class B drug under the Misuse of Drugs Act 1971.