The number of people approaching two Welsh charities with psychoactive drug addictions has risen since they were banned, it has been claimed.
It became illegal to produce, distribute, sell or supply the drugs, formerly known as "legal highs", in May 2016
But substance misuse agency CAIS and Drug Aid Cymru said usage has increased in the last three to six months.
The UK Government said it was continuing to work to stop drugs use.
The charities' comments come after images and camera footage from Wrexham was widely shared in March, showing people who appeared to be under the influence of the drugs.
Clive Wolfendale, of CAIS, said there was more awareness of the substances, meaning more people are reporting use of them.
He added that more research was needed into their use.
"In the past few months in north Wales we've seen more people coming forward presenting with issues of using new psychoactive substances," he said.
"Personally I was always concerned the ban would put the market in the hands of criminals, as we've seen with other drugs like heroin and cocaine.
"My fear is that because it's such a prevalent issue and that substances, irrespective of the ban, are so widely available that this is a phenomenon which will be with us for years, if not decades, to come.
"We see very vulnerable people taking them. It might be that they've fallen prey to criminals, but we need more research to understand what's going on."
Rob Barker, of Drug Aid Cymru, echoed his concerns.
"People using NPS (new psychoactive substances) dipped quite a bit since May, since the law came in which was a good sign," he said.
"The main shops that sold NPS closed, so the accessibility for a short time went down quite a lot.
"But actually, recently in the last three to six months we have started to see people presenting to our services with issues around NPS, particularly around synthetic cannabinoids.
"We are seeing people with quite significant effects and issues associated with these drugs."
Figures from January showed there had been no prosecutions or cautions issued in three of the four Welsh police force areas since the ban.
Minister for Vulnerability, Safeguarding and Countering Extremism Sarah Newton said drugs would not be tolerated.
"Our drug strategy, to be published shortly, will build on the work already undertaken to prevent drug use in our communities and help dependent individuals, including homeless people and those in prisons, to rebuild their lives," she added.