Patients requesting euthanasia will be given unapproved, unregulated and "off label" medicines, sparking warnings of prolonged and distressing deaths.
People who chose to swallow or ingest the fatal medicines, rather than taking them intravenously, would be given drugs that were compounded (mixed up) by a pharmacist and provided to the patient without being approved by regulator Medsafe.
The Ministry of Health said those who opted for an injection would be given drugs which had been approved by Medsafe but for a different purpose - so the medicines will be provided for an unapproved, or "off label", use.
Hundreds of pages of documentation, much of it heavily redacted, has been released under the Official Information Act to RNZ as part of an investigation into how prepared New Zealand is to introduce assisted dying.
Among the documents is an email from Dr Bryan Betty, medical director at the Royal New Zealand College of GPs, warning that mixing concoctions of drugs had led to traumatic deaths.
Betty's warning to the Ministry of Health used the example of American states not being able to access death penalty drugs due to cost and availability.
"So they made up their own concoctions initially, with examples of prolonged processes until fine-tuned. Belgium had a standard process but [this was] not used by many doctors for some years, also resulting in prolonged, distressing deaths."
Betty said it was important to develop strong guidelines to avoid these situations.
"There have been concerns expressed internationally over … the concoction of medication that is used, that in some cases, has led to traumatic end of life experiences.
"I think we need to mitigate those risks upfront and be very prescriptive about what could be used and an end of life situation.
"You need to get it right. And you need to get it right the first time. I don't think we can accept room for error in this space."
Ministry of Health documents released to RNZ show that sourcing medicines was identified as a "key risk" to the assisted dying regime, which was triggered by a 65 per cent yes vote in the 2020 referendum.
"We understand that the medications that are commonly used for the purpose overseas are not available in New Zealand," a policy manager with the ministry, wrote in a December email.
By January, a Ministry of Health implementation project team warned medicines "will not be available on day 1" unless a tight time frame was met, noting that it took up to 18 months to regulate a new medication.
Records of a meeting of Pharmac, Medsafe and ministry officials show international companies could be reluctant to provide the drugs on ethical grounds.
The Ministry of Health's regulatory assurance manager then asked for a literature search on what medicines were used overseas, including "medications given for death sentences" by lethal injection.
The search found there were supply issues as some countries were "banning the export of certain drugs for lethal injection in the US".
Instead, the Ministry of Health now intends to use a section of the Medicines Act 1981 which allows a pharmacist to mix a new medicine after getting a prescription from a doctor.
The ministry says patients opting to ingest end of life medicines orally, or by tube, will be given these unapproved, compounded medicines.
If they opt for the intravenous route then those drugs "are likely to be medicines approved by Medsafe, but for an unapproved [off label] use", a spokesperson for director general of health Dr Ashley Bloomfield said.
Documents released to RNZ show a Ministry of Health literature search on the international experience with euthanasia highlighted problems with using alternative drugs and combinations of medicines.
A 2016 American study said a European Union ban on exporting drugs used for capital punishment had resulted in a drug shortage and "a search for alternative drugs and new drug combinations that have not been previously validated for inducing death".
It said because of this "some of the executions did not proceed as expected", sparking debate about whether these deaths were "cruel and unusual punishment".
Keryn Powell, a Napier GP opposed to euthanasia and concerned New Zealand won't be ready to introduce it, said it was frightening to hear off label and unregulated medicines would be used.
"There would have to be legal cover for any doctors using it off-license because if something goes wrong, who is responsible?
"I think that puts doctors who are performing euthanasia in a very dodgy position.
"How are they actually going to protect the people administering them if something goes wrong?"
Ministry of Health chief medical officer Andrew Connolly said there were international concerns about compound medicines used in euthanasia because it was difficult to test the effects of the drugs.
For that reason injections were favoured in the Netherlands.
Ingesting or swallowing the drugs also may not be tolerated by some patients.
"If we take an example of someone who has widely spread cancer that's affecting the abdomen, their intestines may or may not function as we would need them to function to absorb and deal with the medication."
Secrecy provisions in the New Zealand law will make it difficult for the media to scrutinise which drugs are being used.
Under the End of Life Choice Act, a media company can be fined $20,000 for naming the method used in euthanasia.
Secrecy provisions used in the US have created problems, according to the Ministry of Health literature review.
A 2018 study in the Journal of the American Pharmacists Association looked at how US states were coping with the shortage of execution drugs.
Some had "contracted with small compounding pharmacists to compound them" but what was in them was kept secret.
"By purchasing medicines from non-FDA approved suppliers, and enacting lethal injection sourcing secrecy laws, states are undermining the robust enforcement of chain of custody and pharmaceutical supply chain transparency," the study said.
RNZ has previously questioned what would happen if a patient didn't die in an initial euthanasia attempt.
Health Minister Andrew Little claimed that the issue of failed euthanasia attempts had not been raised with him but the documents released by the Ministry of Health show literature reviews did cover this.
The ministry looked at a 2019 study in the journal Anaesthesia which said that there were no standardised methods for euthanasia resulting in frequent cases of prolonged and distressing deaths.
"With all these forms of assisted dying there appears to be a relatively high incidence of vomiting [up to 10 per cent] prolongation of death [up to seven days] and re-awakening from coma [up to 4 per cent]."
Connolly said the Ministry of Health was still working through what to do in situations where an initial attempt failed.
"There'll be some sort of point in time when they may need to re-consent if it was an abject failure, and the patient … said, 'just leave me alone, I've had enough' sort of thing and so fully regained their faculties," he said.
"How can you re-obtain consent if, say, someone is semi-conscious but has not died? And so we're in discussion with the Medical Council and the Nursing Council about these types of issues. We have to be clear about that - the act is silent on that matter."