Police checkpoint (file photo). Photo: RNZ / Alexander Robertson
The government's roadside drug testing scheme could penalise people using prescription substances, warn drug experts.
The coalition is progressing a law change that will give the police the power to randomly saliva-test drivers for drug use.
Under the proposed scheme, motorists who test positive in two tests will be immediately banned from driving for 12 hours.
All positive tests will go to a lab, and drivers will be fined and issued demerit points if qualifying drugs are found at levels that indicate recent use.
The scheme is still light on detail - such as what drugs qualify and at what levels - but Transport Minister Simeon Brown said the changes will make roads safer.
"This is about setting a line, having a standard and saying that people who have more than that certain amount of that substance in their system are considered to be impaired and we will hold them to account.
"Ultimately, the message is very clear: we do not want people taking drugs and driving on our roads and killing people."
The Labour Party supported drug-testing, having started a similar process last term, but experts have told RNZ roadside testing was not a simple fix.
Massey University senior research officer Dr Marta Rychert - who specialises in drug policy and medicinal cannabis - said the government's policy was light on detail, like how police officers would test if someone was actually impaired by drugs.
Research had shown cannabis typically slowed down users' reaction times, but this did not necessarily apply to regular uses of medicinal cannabis, she said.
"Medicinal cannabis users often develop a certain level of tolerance to the psychoactive effects of THC and therefore they may not be as impaired as infrequent users.
"So despite taking the same amount of cannabis, having the same level of THC in their blood or saliva, they may not actually be impaired. Sometimes they may be okay to work, drive and do all the cognitive tasks that are required."
New Zealand Drug Foundation executive director Sarah Helm uses CBD oil - derived from cannabis - for arthritis to help her gym, play sport and raise her family.
She said herself - and those using other prescription medicines like codeine or tramadol - might be unwittingly be caught in the drug-testing scheme.
"You've potentially got somebody who's taken something according to the prescription. They've followed their doctor's advice, tried to avoid doing anything during the window in which you're impaired... and still get pulled up and put in that driving ban.
"That driving ban would really adversely affect me and I take it without any impairment whatsoever and follow my doctor's instructions."
Helm also raised concerns about the accuracy of the saliva tests as there had been cases in Australia where impaired drivers had been allowed to drive on after delivering a false negative.
On prescription drugs, Dr Rychert said some countries have drug-testing 'exemptions' for people who could both prove they had a doctor's prescription for drugs and were not impaired.
The NZ Drug Foundation supported such a move in lieu of technology capable of distinguishing between drug presence and drug impairment.
Brown said his proposed scheme did not consider where a drug came from.
"The intent of this is not to discriminate between illicit drugs and legal drugs. It's about making sure that people who are on the road are driving safely."
Legislation setting up a drug-testing scheme will go to Select Committee for public feedback after it passes its first reading.
Brown said once it was law, the government will start procuring tests.