Scientists are calling for a major effort to make cannabis use less harmful as a rising number of countries look to replace long-standing and outright bans on the drug with more relaxed legislation.

A number of countries and US states have softened laws around cannabis use against a backdrop of rising drug potency and an apparent increase in the number of people in treatment for cannabis-related problems.

Researchers at King’s College London and UCL said it was now crucial for health officials to consider measures to reduce the harm from cannabis use. Many of the health risks that users face could be reduced by discouraging people from smoking it with tobacco, and using vapourisers instead.

In regions that have legalised cannabis, health interventions could target the drug through caps that enforce a limit on the strength of cannabis, and higher taxation on the most potent varieties. Tinkering with the chemical composition of cannabis could also make the drug safer without altering the pleasurable effects users want, they add.

Writing in the journal Lancet Psychiatry, Amir Englund and other researchers say that with laws around cannabis rapidly changing, the need to protect users from the most harmful effects has never been greater, while more research is urgently needed to inform fresh drug policies.

It is hard to be sure what varieties of cannabis people use and in what amounts because the data tends to be based only on drugs that are seized by the police. But the best estimates suggest that on the global market, cannabis use has on average doubled in the past four decades. In many countries, including the UK, high potency cannabis now dominates sales on the black market.

In the past eight years, the number of Europeans entering specialist treatment for cannabis-related problems has increased by more than 50%. It is unclear how many people refer themselves for treatment and how many are sent to clinics by the legal system, but the scientists believe the evidence that cannabis can cause harm to some users is strong enough to justify efforts to make it safer.

Several European countries have relaxed their laws around cannabis use, including the Netherlands, Spain and Portugal, and in the US eight states have formally legalised recreational use of the drug. Canada is due to follow suit this year. Many other regions now allow the drug to be used for medicinal reasons.

While legalising the drug is expected to cut the costs of cannabis-related crime, it is unlikely to reduce the number of users, the researchers write. “A lot of countries are deciding to change their cannabis policies to more permissive ones and we don’t know whether that will lead to significantly more use and problems, but we do know there are vulnerable groups out there,” said Englund.

The main compounds found in cannabis are THC (delta-9-tetrahydrocannabinol) and cannabidiol (CBD). The THC produces the “high” from cannabis, but greater levels of the chemical are also linked to a higher risk of psychosis. There is some evidence that CBD can combat the negative effects of THC, such as poor memory and paranoia, but in high strength cannabis levels of CBD are often minimal.

One option the scientists propose is to boost levels of CBD in high potency cannabis, so that users can get their hit without being at such risk of mental harm.

“We don’t always have the luxury of waiting for a lot of research and we can sometimes use the scant evidence we have to try and make some kind of best case judgment,” said Englund. “We are fairly confident that higher THC levels in cannabis are not a good thing, and it does seem that higher CBD levels in cannabis are protective, at least to some extent.” Later this year, Englund will launch a study funded by the Medical Research Council to test whether different levels of CBD can reduce the harmful effects of THC.

Ian Hamilton, a lecturer in mental health at York University, said the European habit of smoking cannabis with tobacco was the greatest health risk users faced, but this had largely been ignored by public health. He said there was “a real opportunity to minimise such harms by adapting the strategies that have proved to be effective in reducing smoking incidence and prevalence in the general population.”