Researchers have identified the specific number of weekly delta-9-tetrahydrocannabinol (THC) units beyond which the risk for cannabis use disorder (CUD) increases.
Using standard THC units — defined as 5 mg of THC per unit — the investigators found that consuming more than 8.3 units per week among adults (about 41 mg of THC) and more than 6.0 units per week among adolescents (about 30 mg of THC) represented the optimal cutoffs for increased risk for any CUD.
Higher thresholds — 13.4 units per week for adults and 6.45 units per week for adolescents — were associated with the risk for moderate-to-severe CUD. The UK study, which included adults and teens, showed the accuracy of using weekly standard THC units to identify CUD was high across all models assessed.
Lead author Rachel Lees Thorne, MD, Addiction and Mental Health Group, Department of Psychology at the University of Bath, Bath, England, noted that 8 units per week equate to approximately one-third gram of herbal cannabis on the UK market.
“This will likely be a lower amount than people who use cannabis regularly would typically consume and highlights that CUD can occur even with relatively lower levels of consumption,” Thorne told Medscape Medical News.She added that although the findings may not be generalizable to other settings where cannabis products and use patterns differ, the investigators hope that framing use in THC units could help clinicians have more informed conversations with patients and better track cannabis-related behaviors.
The investigators also noted that theirs is the first study to estimate risk thresholds for CUD based on standard THC units mirroring the way alcohol units are used to calculate higher risk for drinking.
The findings were published online on January 12 in Addiction.
Risk Threshold
About 22% of individuals who use cannabis go on to develop CUD, a pattern of use that leads to clinically significant distress and/or impairment. The investigators noted that in the UK, cannabis use is cited as a problem drug by 87% of patients younger than 18 years who are in drug treatment programs.
A paper published in 2019 proposed that in the US, a “standard THC unit” should be set at 5 mg of THC across all cannabis products and methods of administration.
In 2021, the US National Institutes of Health (NIH) agreed, defining a standard THC unit as “any formulation of cannabis plant material or extract that contains 5 mg of THC.” In its announcement, the NIH added that the definition would apply to any future applications proposing research on cannabis or THC.
In the current study, the investigators used data from the observational CannTeen study of 65 adults aged 26-29 years (54% men) and 85 teens aged 16-17 years (56% women) from London who reported using cannabis at least once during the 1-year study period.
The Enhanced Cannabis Timeline Followback was used to estimate mean weekly THC units by assessing quantity, frequency, and potency of consumed cannabis. A diagnosis of CUD was assessed using the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, with “any CUD” describing a composite of mild, moderate, or severe versions of the condition.
Receiver operating characteristic curve models were used to determine how well weekly standard THC units could distinguish between no CUD and either any CUD or moderate/severe CUD.
Results showed an area under the curve (AUC) of < 0.7 for all models assessing discrimination accuracy of weekly standard THC units on CUD.
For determining no CUD from any CUD, the AUC was 0.79 in the adult-only model and an “outstanding” 0.94 for adolescents. The AUCs were 0.82 and 0.94, respectively, for determining no CUD from moderate/severe CUD.
The optimal risk cutoffs for any CUD were 8.3 units of THC per week for adults and 6.0 units per week for adolescents; for moderate/severe CUD, the optimal risk thresholds were 13.4 and 6.45 units per week, respectively.
Measuring cannabis use with standard THC units “appears to show good discrimination accuracy of [CUD] at different severities and in different age groups,” the investigators wrote.
“Safer levels of cannabis use, defined by low weekly standard THC unit consumption, could be recommended in lower risk cannabis use guidelines,” they added.
‘A Much Needed Start’
In an expert roundup by the Science Media Centre, Marta Di Forti, MD, PhD , Institute of Psychiatry, Psychology, & Neuroscience at the King’s College London, London, England, noted that using this type of standardized measurement could become an “important tool” in both research and clinical settings — in about the same way standardized alcohol units have become.
However, “it is important to remember that cannabis, unlike alcohol, does not contain only one active ingredient but over 144 cannabinoids,” said Di Forti, who was not involved in the current research.
Still, THC units are, “undoubtably, a very important and much needed start,” she added.
David Nutt, DM, head of the Centre for Neuropsychopharmacology, Department of Medicine at Imperial College London, London, England, noted in the roundup that the analysis provided a “welcome update” on recreational THC risks that can lead to dependence.
“What needs to be done now is to facilitate recreational cannabis users in determining exactly how much they are using to help them control their risk,” Nutt said.
“The best way would be through a regulated cannabis market with clear product quality and identification of unit amounts…plus a credible and honest educational program,” he added.
The investigators reported having no relevant financial relationships. Di Forti reported receiving personal fees from Janssen, conducting an education seminar sponsored by Recordati, and delivering a lecture sponsored by Lundbeck/Otsuka. Nutt is chair of the scientific committee of the charity DrugScience, which ran the T21 medical cannabis initiative.
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