The biggest study of its kind shows New Zealand's rate of fatal poisonings remains high, with an average of 234 deaths a year.
And the research involving the University of Otago reveals two prescription drugs for insomnia and schizophrenia are having deadly consequences.
More than 200 New Zealanders a year are being fatally poisoned by carbon monoxide, chemicals and pharmaceutical drugs.
In a study based on coroners' cases from 2008 to 2013, 51 per cent of the poisonings were suicides and 39 per cent were unintentional. The remaining 10 per cent were unknown.
The lead researcher, John Fountain, Best Practice Advocacy Centre Clinical Advisor, says more investigation is needed.
"An unintentional death might occur due to what we'd call an adverse drug reaction - an unexpected event related to the drug. Or it could be a mistake with dosing, or it could be an interaction with other drugs for instance," he said.
The study raises the alarm over two prescription drugs - Zopiclone and Clozapine, which have caused a high rate of deaths compared to overseas.
Zopiclone is for insomnia and is heavily prescribed. Clozapine is for schizophrenia. It has low prescription rates and is heavily regulated. Both these drugs caused 31 deaths each over the course of the study.
Wellington Hospital Emergency Department doctor Paul Quigley says Zopiclone is "always in our top six agents presenting for overdose. But fortunately it very rarely ends in death if you arrive in hospital".
Dr Quigley says the rules need to be tougher.
"We know patients use it every single day and they get three-monthly supplies. So, it's out of hand."
But the drug regulator, Medsafe, says it advises Zopiclone for occasional use and will continue to monitor its safety.
Another substance on Dr Quigley's radar is synthetic cannabis, which he says caused 70 deaths in a single year.
"So that's a third of those deaths we've just have been talking about would be coming from one agent. So that just shows you what a deadly drug it is. And it's by far the deadliest in New Zealand," he said.