The sale of so-called legal highs has gone underground after a blanket ban came into force, a report says.
While the ban has led to a "considerable reduction" in use of the drugs, street dealers are now the main source of supply.
Now officially known as new psychoactive substances (NPS), they mimic the effects of other drugs like cocaine, cannabis and ecstasy.
They were sold openly, mainly in specialist shops, before May 2016.
Since the Psychoactive Substances Act (PSA) came in, they are no longer being sold in the stores, known as head shops, according to the Home Office review.
They said 31 outlets had closed, with a further 332 no longer stocking the drugs. Prices have gone up too and "significantly" fewer people are using them.
But the report warns the drugs have become more potent, with new strands of them continuing to be produced. This suggests the ban has not yet brought an end to the "game of cat and mouse" between law enforcement agencies and those producing NPS.
'You can still get hold of them'
The review also says the ban might have led to the most vulnerable users switching back to drugs like cannabis and cocaine.
One Manchester drug worker quoted in the report said: "It was so easy before. But after PSA, in Rochdale young offenders, those in care, they found they had less access to NPS.
"People just couldn't be bothered to source it and there is a culture of 'whatever they can get their hands on easiest'."
But an NPS user in Exeter quoted in the review said: "Now the shops aren't selling stuff, people get it from another source, like through a dealer. You can still get hold of them just as easily."
Ministers had brought forward the NPS legislation after the substances were linked to dozens of deaths.
It concluded that most of the aims of the act "appear to have been achieved, with the open sale of NPS largely eliminated, a significant fall in NPS use in the general population, and a reduction in health-related harms which is likely to have been achieved through reduced usage".
But it said areas of concern had remained - or emerged - since the ban, and noted "continued high levels of synthetic cannabinoid use among the homeless and prison populations."
The main NPS used within prison is Spice, according to the report.
Under the act, offenders can face up to seven years in prison, with civil orders being issued to close down online dealers and head shops.
Police recorded 492 arrests in the first six months after the act took effect. There were 1,523 seizures of NPS in 2017/18.