Researchers at the University of Bath in the UK are proposing thresholds for safe – or at least safer – cannabis use and hope their findings will help people monitor consumption and keep it within recommended limits – similar to how alcohol units guide safer drinking.
The threshold recommendations, proposed in a paper published today in the journal Addiction, are based on a system for measuring cannabis consumption not by weight but by THC content (THC is the compound responsible for the psychoactive effects of cannabis).
In the same way as guidelines for safer alcohol use focus on standard units (e.g., adults are advised not to regularly exceed 14 alcohol units per week in the UK), the researchers propose that a similar unit could be applied to cannabis.
The aim of this work is for people who use cannabis as well as clinicians and public health bodies to prioritise THC units – which reflect both cannabis potency and the amount consumed – rather than relying solely on frequency of use.
The findings recommend that adults should not exceed 8 THC units per week – equivalent to about 40 mg of THC or 1/3 gram of herbal cannabis.
Above these thresholds, people are at greater risk of developing cannabis use disorder (CUD), a condition that affects an estimated 22% of people who regularly use cannabis . This is a problematic pattern of cannabis use causing clinically significant impairment or distress. Symptoms can include cravings, struggling to control use, and cannabis interfering with work, family or other relationships.
In the new study, Dr Rachel Lees Thorne and Professor Tom Freeman – the lead and senior researchers from the Department of Psychology at Bath involved in this work – have applied this unit to establish, for the first time, thresholds at which cannabis could be considered 'safer'.
Dr Rachel Lees Thorne said: "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. For instance, a person might opt to use lower-THC products or reduce the quantity of cannabis they use.
"These guidelines are designed to offer realistic, evidence-based advice for those who want to make informed choices."
When cannabis use reaches a higher risk level
The new research drew on data from the CannTeen study conducted at UCL, which tracked 150 people who used cannabis, assessed the severity of CUD and estimated their weekly THC unit intake over a year.
The team established that for adults, the risk of CUD increases above 8 THC units per week, with the risk of more severe CUD rising above 13 units per week. In the CannTeen sample, 80% of people who used below 8 THC units did not have CUD, while 70% who used above this amount reported CUD.
Professor Freeman said: "Cannabis is one of the most widely used drugs in the world. Despite this, there is no information for consumers about how different levels of consumption might affect them.
"Safer use thresholds based on standard THC units could help people better understand their level of use and make informed choices about their health. Such thresholds could be used by public health bodies and in healthcare settings for communicating the risks of an individual's level of consumption, and for tracking reductions in use.
"As cannabis becomes increasingly available in legal markets around the world it is more important than ever to help consumers make informed choices about their use."
Building on these initial findings in a UK sample, the team plans to look at safer cannabis thresholds across larger international samples, and to develop tools to help people track their unit consumption in different international contexts.
Better labelling in countries where cannabis is legal
The Bath research is already attracting international interest, particularly in countries with legal cannabis markets, such as Canada, where there is growing momentum to include THC unit information on product labelling, in the same way as alcohol products are labelled with alcohol units in the UK.
The Canadian Centre on Substance Use and Addiction is leading a global working group on cannabis units, and the Bath team has shared its findings to support this work.
Dr Robert Gabrys, senior research and policy analyst at the Canadian Centre on Substance Use and Addiction (CCSA), said Canada's expert panel for the legislative review of the Cannabis Act has made it a priority to develop a 'standard dose' for cannabis products.
He said: "Cannabis legalisation in Canada has brought a much wider range of products to the market. With that, many people face challenges understanding product labels and how to safely dose their cannabis products. This has led to the need for more effective approaches to help people interpret product information and better understand the potential health effects of their cannabis use.
He added that the research from Bath, "is an important part of this effort, as it demonstrates how a standard THC unit can be used to predict and communicate health risks – in this case CUD."
