The ban on new psychoactive substances, including the “zombie drug”, spice, has served to drive the trade underground as more potent and unpredictable strains enter the market, pushed by street dealers.

“Personally, I think the situation has got worse,” said Arfon Jones, the police and crime commissioner for North Wales, which includes the town of Wrexham, whose problems with spice have been highlighted by local residents on social media.

“The psychoactive substances act has had unintended consequences in sending the problem underground. I am not saying everything was hunky dory when we had head shops selling on the high street, but the government missed a trick in not regulating the head shops, rather than prohibiting the psychoactive substances.”

Surveys suggest that the number of people using the drugs – known as synthetic cannabinoids – declined after the government introduced the Psychoactive Substances Act last year. Among 16- to 59-year-olds, usage has decreased from 0.7% in 2016 to 0.4% this year, driven by a significant reduction in use among young men, from 3.6% to 1.6%.

But there are concerns that spice – now a class B drug – and its variants are mutating.

“What we’ve seen particularly with the synthetic cannabinoids is a reduction in the range that appear to be available on the market, and those that are available are stronger and more dangerous – potentially fatal,” said Josie Smith, head of the substance misuse programme at Public Health Wales.

There are fewer on the market but they are considerably more toxic and dangerous which we can only presume was an unforeseen consequence of the introduction of further drugs legislation.”Smith oversees Wedinos, the Welsh emerging drugs & identification of novel substances project, which tests street drugs for users. She said: “These are drugs that are causing immeasurable harm, not only to users but across the piste in terms of health and social and criminal justice services.

“They have been around for a few years, but the compounds and combinations have evolved and are more dangerous and more potent.”

Prior to the ban, these substances were sold mainly in head shops – retailers specialising in drug-related products. But after the law changed, the shops stopped selling the product and many closed altogether. An illicit trade emerged, targeting prisoners and the homeless.

Professor David Nutt, former chair of the Home Office’s advisory council on the misuse of drugs, said the effects of the ban were “exactly as predicted”.

The range of synthetic cannabinoids that are available are stronger and more dangerous – potentially fatal. Josie Smith

“Head shops have stopped selling spice, so it’s now being sold on the black market. There’s less quality control and very little guidance [on their use]. I always argued that people who were selling synthetic cannabinoids were aficionados who sold what they liked, and gave their customers some guidance. We have lost all of that safety element now. It’s all underground and the products are very cheap, so we see the homeless targeted.”

Public health services are reportedly overwhelmed by call-outs to spice victims found slumped unconscious in public spaces. Smith said that, in a recent conversation with one ambulance service, she learned that they had been called out five times in the space of an hour.

“We were saying before the ban that it wasn’t going to solve the problem and that’s clearly still the case,” said Rick Bradley, operations manager at drug treatment charity Addaction.

“There have been some changes, maybe the more experimental use has died away, but among homeless people, young people in care and prisoners, the use is still continuing. The vulnerable populations who are continuing to use it are now marginalised and are at even greater risk.”

A Home Office spokesperson said: “We recognise how dangerous synthetic cannabinoids such as those sold under the brand name Spice can be and the devastating impact that they can have on our communities, families and the individuals taking them.

“That is why we banned these so-called legal highs. Since we introduced the Psychoactive Substances Act we have seen use of these substances fall significantly, hundreds of retailers shut, and the first offenders convicted.

“We have also acted by reclassifying the drugs sold as Spice as class B to give the police the powers they need to take action, including making possession illegal and delivering longer sentences for dealers.”