Lawmakers say their hands are tied in their attempts to ban powerful synthetic cannabis products.

Associate Health Minister Peter Dunne said he would liked to have seen the substances prohibited but New Zealand's outdated drug laws prevented it.

"If it looks like cannabis, smells like cannabis, we ought to be able to treat it like cannabis," Mr Dunne told the Taranaki Daily News yesterday.

The Expert Advisory Committee on Drugs, after researching and testing the products, recommended they be classified R18.

Mr Dunne said the committee had two options – to classify the products as a restricted substance or as a controlled substance.

"They said they were dangerous but in their conclusion they were less harmful than cannabis – the real thing," Mr Dunne said.

"They weren't confident they had sufficient evidence to justify them being made a controlled substance."

Mr Dunne said he had no option but to accept the recommendation.

"I would have preferred to go stronger. The evidence was simply not there to go beyond that.

The worst case scenario would be if he imposed a total ban, which was then challenged in court and not upheld due to lack of evidence, he said.

"In effect that throws out all the law. I didn't want to risk that."

He said the current legislation was very old and written before the substances were available and hoped a Law Commission review of the Misuse of Drugs Act 1975, due out in about a month, would add more teeth.

"I would like to see the law made much more explicit, so that these drugs and others like them that come along can be banned outright," Mr Dunne said.

He wanted authorities to have the power to take a much more sweeping approach to drugs.

"The one thing that I didn't want to risk is for someone to argue that given this classification, maybe cannabis should be classified as a softer substance."

New Plymouth MP Jonathan Young wanted the R18 classification reviewed, saying it promoted a soft stance on dangerous drugs.

"Personally I'm very unhappy that, as a synthetic cannabis, we would consider it to be R18 because that then does create opportunity for natural cannabis to be pushed into that category," Mr Young said. "It undermines our line regarding drugs."

He wanted a decision based on robust evidence but urged a cautious approach.

"Suggesting it be an R18 product at this stage opens up a Pandora's box that might prove very difficult to close," Mr Young said.

The synthetic dope products are freely on sale in New Plymouth dairies, displayed next to chocolate and chips. They sell for $25 a packet, which contains enough material for three or four joints.

They contain a blend of synthetic and natural cannabinoids and produce similar effects to THC, the substance in cannabis which gives users the high.