• PM under fire on funding of cancer drug
• Says debate is "very emotional"
• Drug used by Jimmy Carter to treat brain cancer

John Key is not ruling out overriding Pharmac to fund cancer drug Keytruda, but says choosing which drugs to fund is a "balancing act".

The Prime Minister, appearing on the Paul Henry Show this morning, said the debate around the funding of Keytruda, or, was "very emotional".

The drug is used to treat melanoma and is considered by some cancer specialists to be the greatest advance since chemotherapy. It is state funded in both Australia and Britain.

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Mr Key told Paul Henry the drug could "potentially" be funded, but said he preferred not to override Pharmac.

"I can't say we'll override the ruling from Pharmac, our preferred option is always for Pharmac to make the right decision and get there. It's very difficult ... it clearly works for some people and for others it doesn't."

He added: "What they are saying is, on the budget they have at the moment in theory there are some other drugs that are better."

The drug could be funded if health was allocated more money, he said.

"We can't override Pharmac easily, but if we give them more money they might."

When questioned by Henry about the potential of patients dying while waiting for Keytruda to be funded, Mr Key responded, "Trust me when I tell you we want to help these people. We desperately want to save them. There are lots of drugs and lots of them are expensive."

He said scientific advances meant new drugs were being released rapidly, which meant Pharmac had to make difficult decisions about what to fund.

Former President Jimmy Carter discusses his cancer diagnosis during a press conference at the Carter Center earlier this year. Photo / Getty
Former President Jimmy Carter discusses his cancer diagnosis during a press conference at the Carter Center earlier this year. Photo / Getty

"I know people will say we did that for Herceptin and we did, we campaigned on it and we did it.

"If that's the way we make the decision, then by next year, because of the wonders of modern science, there will be another drug that will come along. Not necessarily in melanoma, but in something else. That's the balancing act here."

Pharmac's experts committee last week granted it a low-priority status because of uncertainty about its benefits and its high cost -- about $300,000 a patient for two years' treatment.

Former US president Jimmy Carter this morning announced he was cancer-free, following treatment with Keytruda.

Labour leader Andrew Little and health spokeswoman Annette King have pledged to fund Keytruda if Labour is elected in 2017.

Mr Little told the Herald last night that in the meantime, the Government should direct Pharmac to fund the drug, which is considered by some cancer specialists to be the greatest advance since chemotherapy.

It is state funded in Australia and in Britain.

Pharmac's experts committee last week granted it low-priority status because of uncertainty about its benefits and its high cost -- about $300,000 a patient for two years' treatment.

Mr Little said there was a case for political intervention into areas of clinical judgment.

"You set up a system with a level of independence for obvious reasons for clinical judgments but they are clinical judgments for resource allocation.

"I think there is a case for politics where the circumstances are such you've got widespread incidence of the condition that you've got a drug for and it is used in other countries.

"If the reason it is accorded low priority is because they are awaiting more data, I think those are circumstances where politics can intervene and say 'we are not going to deny this to New Zealanders pending more data'."

Paul Smith, New Zealand director of Merck Sharp and Dohme which supplies Keytruda, said 70 per cent of patients either had tumours shrink or stabilise with treatment.

Melanoma is New Zealand's fourth most commonly registered cause of cancer with 2300 new cases each year. Three hundred and fifty people die of it each year.

National politicised the funding of Herceptin in the 2008 election campaign, promising to fund a 12-month course of the dug used in the treatment of breast cancer instead of the nine-week treatment Pharmac was previously funding.

Pharmac's combined pharmaceutical budget for the year ended June 30

• 2015 - $795 million
• 2014 - $795 million
• 2013 - $783.6 million
• 2012 - $777.4 million
• 2011 -$706.1 million
• 2010 - $693.8 million
• 2009 - $653 million
• 2008 - $635.4 million