The two most notorious synthetic cannabinoids to hit New Zealand, linked to dozens of deaths, have all but vanished in the space of a year.

The sharp drop in local detections of "zombie" chemical AMB-Fubinaca and the similarly potent 5F-ADB has been put down to tough new law changes here and in source country China.

But they appear to have been replaced in New Zealand's black market by new and structurally similar forms of the so-called "synnies".

AMB-Fubinaca - reported to be 75 more powerful than the THC in cannabis plants - was particularly infamous for inducing zombie-like behaviour, and fuelling an outbreak in New York in 2016 that gained it worldwide notoriety.

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Many people have also died smoking 5F-ADB. Its part in one young man's death in January 2018 prompted a warning from Rotorua coroner Dr Wallace Bain.

Since ESR began surveying them in 2017, AMB-Fubinaca and 5F-ADB respectively accounted for 89 per cent and 77 per cent of synthetic cannabinoid detections between May 2017 and March last year.

Over the same period, toxicological evidence linked AMB-Fubinaca to 64 deaths in New Zealand - most of them in the North Island.

One newly-published study into an AMB-Fubinaca-driven outbreak of deaths, that began in Auckland in mid-2017, found about nine out of 10 deaths were men, with a mean age of 42.

Nearly a third had mixed the substance with other drugs, around half suffered heart disease, and the same proportion had been diagnosed with mental health conditions.

The Government responded to the threat by reclassifying the two synthetics as Class A Drugs, giving police greater powers to search and seize them, and the courts tougher sentencing options.

But ESR scientists have now reported a "complete absence" of AMB-Fubinaca in New Zealand, with cases of 5F-ADB also scarce.

"There's been a significant decrease in the presence of AMB-Fubinaca and 5F-ADB since the end of 2018, for a number of reasons," an ESR spokesperson said.

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They included local law changes and media attention - but the biggest impact appeared to be a crackdown in China, where much of the supply had been flowing from.

In August 2018, China included the two compounds in reforms to its national drug legislation.

It's not the first time drug law changes in China have had flow-on effects elsewhere.

After China legislated against another synthetic linked to deaths and seizures, AB-Chiminaca, in September 2015, New Zealand officials soon began seeing less at the border.

This still from a police video, taken in 2017, shows a man intoxicated near Auckland's City Mission after using synthetic cannabis. Photo / Supplied
This still from a police video, taken in 2017, shows a man intoxicated near Auckland's City Mission after using synthetic cannabis. Photo / Supplied

"The knock-on effect has shown a complete shift in the types of synthetic cannabinoids detected in both border and domestic seizures," the ESR spokesperson said.

"Although there appears to be a lag time between the drug control in the source country and the effects in the importing country."

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Those synthetic cannabinoids being detected currently were structurally similar to 5F-ADB, and believed to be the result of Chinese synthesis labs replacing certain precursors to manufacture slightly modified substances.

"This hypothesis is consistent with the increase in detections of 5F-MDMB-Pica and 4F-MDMB-Binaca, which are both very similar in structure 5F-ADB, in New Zealand."

In the most recent ESR survey, 5F-MDMB-Pica made up about half of all detections, while 4F-MDMB-Binaca made up 22 per cent.

There have also been cases of drug-makers combining AMB-Fubinaca with pFPP, a piperazine derivative sold as an ingredient in psychedelic party pills here and overseas.

Scientists suspected designers were either trying to make their highs longer and stronger, or less harmful.

And in February, drug-testing service KnowYourStuffNZ put out a warning after 5F-ADB was found in a substance, dubbed "crystal LSD", began circulating around Auckland.

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Over recent years, synthetic recreational drugs had become such a problem that Wellington's Capital and Coast District Health Board launched a pilot programme to try to pin-point new ones that patients had taken.

Customs statistics show around 24kg of synthetic cannabinoids have been intercepted over the past decade – with most of the 176 different items coming through the mail centre.

Police say while volumes have been steadily decreasing, they were still causing harm in Kiwi communities.

"There were still 37 recorded hospitalisations related to synthetic cannabinoids in 2019," Detective Inspector Blair Macdonald, manager of the National Drug Intelligence Bureau, told the Herald.

"The threat of another synthetic cannabinoid outbreak is very real and in part led to the development of New Zealand's first drug early warning system, High Alert."

The website Highalert.org.nz provided the public with alerts and notifications when dangerous substances were identified.

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"It also allows the public to report unexpected or concerning effects from drugs, and provides access to harm reduction information."