The Prime Minister says low income earners should not be worried about the higher price of prescriptions because they too will benefit from improvements to other parts of the health service.
The Government has said the $40 million a year gained by the move will be used to fund services such as cancer treatment and elective surgery.
From January 2013, the cost of a prescription will increase from $3 to $5 up to a maximum of 20 items a year. Prescriptions for children under six years old will remain free.
Opposition parties say it is callous and will put the cost of medicine out of reach for many.
But John Key says low income earners should not be worried about paying more for medicine.
Health Minister Tony Ryall says he does not want to put up charges willy-nilly, but the rise is an inflation adjustment and the first to prescription charges in 20 years.
Mr Ryall told Radio New Zealand's Morning Report programme on Tuesday that many people, including those with low incomes, are facing cost of living increases.
"Like cost of living increases, if things are putting people in a position where they can't afford these sorts of things they can go and see Work and Income New Zealand.
"We know the Pharmaceutical Guild has talked about various funds that PHOs (primary health organisations) operate, so there's already an infrastructure of support for those who may face financial difficulty."
Mr Ryall says the Medical Association believes it is a relatively minor change that will not be a barrier to health care.
Rise could lead to hospitalisation - doctor
However, a family doctor in Northland says the rise in prescription charges will make life more difficult for many patients and they may end up in hospital.
Kaitaia GP Lance O'Sullivan says many low-income patients - even those with jobs - struggle to see a doctor and get their medicine.
"They get in to see us and I can see their eyes roll in the back of their head when I say I need to give them medication. The first question they ask is, 'Is that free and how much is it going to cost me?'"
Dr O'Sullivan believes the increase will put people on low incomes off from visiting their doctor. He says even at $3 per item, he encounters up to three cases a week where patients can't afford to pay for their medication and worries what an increase to $5 will mean for those on low incomes.
"In a community with high needs, even $3 per item is a significant barrier and it seems ridiculous that you'd need to check if someone's got $3 in their pocket. But there are times when people do not have that money to go and get the items they need. You may be talking one or two and in some cases 12 items, which adds up pretty quickly."
Dr O'Sullivan disagrees with the Government using the money to pay for other parts of the health sector because it advantages one group at the expense of another.
But Bev O'Keefe, the chair of the General Practice New Zealand group which represents networks of doctors' practices, says it is a modest rise of $2 that could have been expected.
"While $2 is hard for some families to find, if medicines are a priority, then we would hope that for most families this is not going to be a significant issue."
Dr O'Keefe says she believes those who can't afford the increase will be the same people who can't afford it now and assistance should be available for them.
Mana warns children at risk
The Mana Party says doctors are already reporting that cost will be a barrier to people seeking medical treatment and leader Hone Harawira says if the price of medicine goes up, children in poor families will die as a result.
"Doctors are saying right now that children's health is being threatened by the price of medicine now. You'd have to assume that if the Government raises that price, that children will die as a result of that measure. I can't believe that any government could be so callous."
New Zealand First says it will affect the young, the poor and the elderly the hardest. Leader Winston Peters says this policy has come out of the blue, as National was silent on its plans to increase the cost of prescriptions during last year's election campaign.
"This is straight out of left field and that's put out now so that what is going to be a pretty black Budget looks less bleak come the announcement on Budget day."