Kiwis continue to turn away from legal highs after a government crackdown - but those who continue to use face increased health risks from "gangsters" selling the unregulated products, drug experts warn.
Data from the Global Drug Survey (GDS) released this week shows use of novel psychoactive substances, often known as legal highs, has steadily dropped in New Zealand since a 2014 law change.
Worldwide, use of "legal highs" increased in the 2016 survey: 4.8 per cent of respondents reported a purchase in the last 12 months, compared with 4.2 per cent in the 2015 survey.
However, New Zealand bucked the trend, with only two per cent using a legal high - down from 4.7 per cent in 2015, and 8.7 per cent in 2014.
Over 7000 Kiwis took part in the survey, which was self-selecting.
Parliament banned all legal highs in May 2014 until they could pass a testing regime, reversing interim approval it had given for low-risk products as part of the Psychoactive Substances Act.
The survey's authors said the findings were an indication that closing "head shops", where legal highs were sold, could lead to reduced sales "with probable displacement to the traditional illicit drugs market".
Massey University illicit drug researcher Dr Chris Wilkins said the university's Illicit Drug Monitoring System report, an annual survey of frequent drug users, had also found a decline in the use and availability of synthetic cannabinoids since the legal highs ban.
Wilkins said the fall of synthetic cannabinoids coincided with a "recovery" in use of cannabis, suggesting a link between the two.
However, he dismissed suggestions that people were using harder drugs instead.
"I've heard some stories that, 'Oh, people are going to go back to meth', but I don't honestly believe that those two things are particularly good substitutes.
"People might go back to cannabis, they might just go back to drinking more alcohol."
Wilkins said while the ban could be seen as a good thing, given some of the synthetic cannabinoids previously sold were "pretty terrible", there was a risk that a lack of regulation meant more dangerous products could turn up in New Zealand.
GOVT 'HANDED MARKET TO GANGSTERS'
NZ Drug Foundation director Ross Bell said the survey's findings were complicated by the fact that "legal highs" were no longer legal, whereas they could still be bought over the counter in other countries.
"There's a group of people who can no longer walk into the dairy or Cosmic Corner or an adult store and buy these things ... [and] won't then try to find these synthetic cannabis products on the black market.
"But we also know that people are sourcing these things on the black market ... smoking synthetic cannabis products which are pretty much knocking them out first thing in the morning."
Bell said some Kiwis would have stopped using drugs altogether, some would have started buying the legal highs from the black market, and others would have turned to more traditional illicit substances.
He believed the ban on legal highs had been a failure, increasing the chances of harm for those who continued to use the substances by reducing the safeguards in place.
"Right now on the black market is a whole lot of new, unknown and, I reckon, quite dangerous substances.
"The Government handed over the ability to have a controlled and regulated market, and they've handed that ability to the gangsters - they've abdicated responsibility."
MARKET 'DRIED UP'
The death of a Kapiti man from a suspected overdose of the drug known as bath salts was a sign the current system did not work, he said.
"These are people that are already choosing to use drugs, no amount of drug education is going to stop them using, they've already made the choice, so how can we keep them safe and how can we stop them dying?
"In terms of bottom lines, I think that's fairly legitimate."
Associate Health Minister Peter Dunne said the effective ban in New Zealand meant the country was not receiving the "flood of psychoactive substances" that other countries had to deal with.
"My understanding ... is that the market has pretty much dried up."
Dunne conceded that those who continued to use the substances were at greater risk because of the lack of regulation, but said the drop in use meant very few were affected.
"I'm not saying that we shouldn't be concerned about the consequences, all I'm saying is that the numbers have dropped off and are very small."
It was "hard to see" any psychoactive drugs being approved for sale soon, given a ban on animal testing to prove their safety.
However, Dunne believed that could change if pharmaceutical companies faced a similar ban and had to develop alternative testing regimes.