A ban on synthetic drugs has pushed the product underground and led to a black market operating, says a health researcher.

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Marta Rychert has looked at the implementation of the Psychoactive Substances Act five years ago as part of her thesis for Massey University. It has been published in the International Journal of Drug Policy.

Dr Rychert said the implementation of a regulated market for synthetic drugs and a subsequent ban on many of them was rushed, and without proper consultation.

More than 20 people have died in recent months after taking synthetic drugs, she said.

"They are still available ... so what it means is that there are no regulations now ... no one is going to check if you are 18 or 20 when you go to the drug dealer on the blackmarket ... so we can't control these drugs ... we've given the power back to the blackmarket," she said.

Dr Rychert said the regulation of these products needed to be better thought through, before the referendum on legalising cannabis, which is due to be held in 2020.

Data from the Justice Ministry shows there were 172 convictions for synthetic drugs last year.