The Institute of Environmental Science and Research says people taking prescription medicines for conditions such as ADHD are unlikely to be caught out by roadside drug testing of drivers.
The ESR currently carried out drug and alcohol tests on blood samples submitted by the police, Ministry of Justice and the NZ Transport Agency.
It said there would need to be a law change and a consensus of how screening would be done, before the introduction of a roadside test for drug impaired drivers.
The agency's forensic toxicology and pharmaceutical manager, Mary Jane McCarthy, said current legislation required that a driver had to fail a compulsory impairment test before a drug test could be required.
Members of the public have contacted RNZ to suggest random roadside drug testing of drivers would be perilous for those on certain prescribed medications, which can show up in their system as methamphetamine.
"I take my ADHD medication in the morning and test positive for meth while driving a few hours later, can the law accommodate this?", one man wrote.
Dr McCarthy said until it was known what type of technology might be used in random roadside testing in New Zealand, it was difficult to say whether this could happen.
"There are quite a number of different types of testing that you can do at the roadside, but you'd need to be a little bit more specific in terms of the kind of testing you're wanting to do."
She said it was unlikely that illicit drugs would show in the saliva-based random test such as that used overseas.
Police Minister Stuart Nash told RNZ last week that saliva-based roadside testing was used overseas, and that New Zealand was looking at new technology that was evolving all the time.
Dr McCarthy said at the moment, a number of different biological samples could be tested for the evidence of drug use, however, the presence of drugs did not necessarily mean that the person was impaired by the drug.
"Blood is the best sample for analysis, however, taking a blood sample or urine sample would be impractical and possibly considered by some to be too invasive."
She said that while alcohol, cannabis and methamphetamine were known to be commonly used, and of concern in relation to road safety, there were many other drugs that could affect a person's ability to drive safely.
The ESR was currently working with a New Zealand biotech company on developing technology, which could quickly and efficiently test drug-impaired drivers using saliva.
Dr McCarthy said the technology could be used to detect specifically targeted drugs rapidly, but that would still require a law change before it could be administered to drivers.
The technology, developed by AuramerBio, uses synthetic DNA that is programmed to recognise specifically targeted drugs.