Even just a few joints a week can make a difference ― measured in milligrams of THC. A new study by researcher Rachel L. Thorne and her team provides the first concrete thresholds for the weekly dose at which the risk of cannabis use disorder increases significantly. Thorne is a research associate in the psychology department at the University of Bath, UK, whose areas of expertise include cannabis use and its consequences for youth and adult health. The study was published on Monday in the journal Addiction.

How much THC per week poses a health risk?

The researchers used data from the CannTeen study involving 85 adolescents (aged 16 to 17) and 65 adults (aged 26 to 29) who had used cannabis in the past year. One THC unit, defined as 5 milligrams, is analogous to the standard unit used to compare beer, wine and spirits in alcohol research.

Based on surveys of consumption patterns and a clinical diagnosis at the end of the study, the researchers derived thresholds that mark the difference between unremarkable consumption and cannabis use disorder.

A cannabis use disorder is present when someone can no longer control their cannabis consumption and continues to use despite clear problems in everyday life. Typical signs of the disorder are neglecting responsibilities at school, work or with family, and withdrawal symptoms such as restlessness or sleeping issues when trying to quit.

For adolescents, this threshold was around 6 THC units per week ― i.e., around 30 milligrams of THC ― and for adults, it was around 8 units, or around 40 milligrams per week. For moderate to severe disorders, the values were higher. The research team emphasized that only abstinence is completely risk-free.

THC units based on alcohol research

In alcohol research, consumption is also generally measured in standard drinks or units, and thresholds for “risky” behavior, like binge drinking, have been established.

“Threshold values are generally very useful for communicating health risks,” says Jakob Manthey from the Centre for Interdisciplinary Addiction Research at the University of Hamburg.

But there is also a risk that such values can be misinterpreted.

“There is a danger that consumption below the threshold value will be interpreted as harmless or even beneficial to health,” says Manthey, who was not involved in the study.

Unlike alcohol, however, cannabis contains many active ingredients whose interaction influences the drug’s effects and risks. Although THC is indeed the most important risk factor, other cannabinoids produced by the plant, as well as the form of consumption ― whether via a joint, vaporizer or edible ― can significantly alter both dose and effect.

How reliable are the THC figures in the study?

One of the study’s strengths is that the researchers repeatedly asked the same individuals about their cannabis use over the course of an entire year. However, the sample size is small (150 individuals), and the actual THC content of the products consumed had to be estimated from external sources, as no lab analyses of individual samples was performed.

The figures the study quotes should therefore be understood as initial guidelines rather than hard limits. But they show, unsurprisingly, that the higher the weekly THC intake, the greater the risk of cannabis use disorder becomes.

Benefits for diagnosis, therapy and prevention in cannabis use

The new thresholds do not replace doctors or therapists for diagnosis and treatment, but they can help with preliminary screening. Specialists could, for instance, begin asking those affected how many THC units they consume per week in order to better assess risk and detect a potential disorder at an earlier stage.

In doing so, they would be following guidelines for the treatment of cannabis-related disorders, which state that frequency and quantity of consumption ― as well as the potency of cannabis consumed ― are important risk factors.

A standardized unit system could help make this information more comparable in the future. It will not, however, change consumption patterns all on its own, since availability, advertising and measures such as youth protection and advertising restrictions play a major role in that as well.

What THC units can do ― and what they can’t

Practical applicability remains a key problem: Many consumers simply don’t know how much THC their products contain, especially if they are home-grown or sourced illegally.
“Under the current regulations, there will be no widespread communication of THC units, so consumers often have no reliable way of knowing the THC content of the products available,” says Jakob Manthey.
British neuropsychopharmacologist David Nutt still views the new analysis as an important step.
“The data provide an estimate of a threshold of weekly consumption to eliminate dependence risk,” Nutt said.
He is calling for “a regulated cannabis market with clear product quality and identification of unit amounts (as required for alcohol currently).”
The proposed THC units would clarify previously vague terms like “a lot” or “risky” when it comes to cannabis consumption. But those wanting to protect their health need more than just a weekly unit number. They also need honest information about potency, effective prevention and, when in doubt, the willingness to limit consumption or quit altogether.

This article was translated from German