One of the scientists who is designing the testing regime for synthetic drugs says trialling novel drugs on humans without testing them on animals first is likely to be considered unethical in New Zealand.
Government will again debate whether novel psychoactives should be tested on rodents, rabbits and possibly dogs when they debate drug laws under urgency next week.
The law already requires legal highs manufacturers to use non-animal tests if they are available. But the Green Party wants it to go further and will seek an amendment to rule out all testing on animals.
Dr Malcolm Tingle, a toxicologist who is helping to design the testing regime, said international guidelines for medicines and cosmetics said that a product usually needed to pass five thresholds to be approved as "low-risk".
These thresholds were low addiction potential, low risk of harm to an unborn baby, low potential to cause cancer in later life, no cumulative effect after repeated use, and no likelihood of death after a single dose.
Dr Tingle, who was speaking in a personal capacity, said animal trials were necessary for some of these thresholds — such as the effect on unborn babies and system toxicity. Most of the other requirements could be trialled with non-animal alternatives.
Asked about whether psycho-actives could be tested first on humans without animal trials, he responded: "A Health and Disability Ethics Committee ... is likely to want to see some pre-clinical evaluation to assure them that volunteers will not suffer adverse effects.
"Without that assurance, the absence of animal testing shifts the ethical dilemma to human ethics committees and they are thus unlikely to approve any such study."
Another expert said it would be possible to test psychoactives without animals if they were made of a known chemical structure.
Professor Nick Holford, of the University of Auckland's pharmaceutical and clinical pharmacology department, said: "This would ... be on a case by case basis. If a new molecule from a well established and acceptably safe class was proposed for use in humans then it would be reasonable to go directly to humans for further testing of effectiveness and safety."
The Green Party bid to block animal testing appears likely to fail.
National would require support from New Zealand First or Brendan Horan to oppose the amendment because its coalition partners Act and Maori opposed animal testing.
New Zealand First leader Winston Peters said the party would discuss its position at caucus on Tuesday, but he wanted the testing regime to be as robust as a pharmaceutical testing system.
Mr Horan said the animal testing amendment was a distraction.