Addiction experts say standard units, similar to those used for alcohol, would help consumers know the level of drug they are taking and its effects

Standard units for grading the potency of cannabis – similar to those already used for alcohol – would result in significant improvements in the mental health of users, according to addiction experts.

Researchers from the Addiction and Mental Health Group at the University of Bath, working with staff from King’s College London, UCL and the Australian Catholic University in Melbourne, say more needs to be done to make people aware of the levels of THC – the main psychoactive component – in the cannabis they are consuming.

Writing in the journal Addiction, the experts suggest a unit level should be set at 5mg of THC – the amount that would typically be found in a small joint. This is enough to induce intoxication but without psychotic symptoms, they say.

“Our findings add to a growing body of evidence that suggests the health effects of cannabis are dose-related,” said lead author Sam Craft, from King’s College London. “We believe a unit system would help both users and healthcare professionals by providing clearer information on the types of cannabis products and their strength.”

Previous research from the team has highlighted how concentrations of THC in cannabis have doubled across Europe in the past decade. But although use of the drug is widespread and there are moves in some countries to legalise it, standard units – which have been commonplace for alcohol for many years – have not been adopted in health guidelines.

The authors believe that a standard unit system would also yield benefits for countries where cannabis use is illegal, such as the UK. “Where the unit system for alcohol has helped consumers to better manage their alcohol intake, so, too, this could have important implications for cannabis users,” said senior author Dr Tom Freeman from the University of Bath. “This should give clear guidance about the dose of THC people are consuming. Our hope is that the introduction of a system in locations where the drug is legalised will have knock-on effects to countries where it is not, providing users and clinicians with an important toolkit to guide safer use.”

The publication coincides with a second study from the team, published in the journal Psychological Medicine, which examines the relationship between cannabis and key health outcomes in more than 55,000 people across 175 countries.

Using the Global Drug Survey, consumers were asked about the types of cannabis they used and their health. The results showed those who used higher THC forms of cannabis, such as hashish, experienced more severe health problems than those using traditional herbal products with lower levels of THC.