Chris Wilkins, head of the drug research team at Massey University's Centre for Social and Health Outcomes Research and Evaluation, said any drug making it on to the market could be a potential health hazard.
"The great hope and the intention [of the legislation] is that the arrival of this new regime will get safer legal highs on the market and ban the other ones. But that's not going to be easy identifying the safer ones.
"Is there such a thing as a safe legal high? People tend to find out they're unsafe only after they've been using them for a while."
Wellington emergency medicine specialist and toxicologist Paul Quigley said it was "stupid" that people had to be harmed before products could be proven unsafe.
He saw patients with bizarre withdrawal symptoms and insomnia after taking party pills, and surges in admissions that had led to contaminated products being banned.
"Instead of closing the door once the horse has bolted, it's not opening the stable door to start with.
"If [manufacturers] really believe in this stuff, then they would prove how safe it is. I would be surprised if any of them actually step up and put their product under that level of scrutiny."
