The Government is being warned that New Zealanders' lives could be at risk if the Trans Pacific Partnership (TPP) deal makes medicines more expensive.
Negotiators from the 12 countries in the talks are meeting in Hawaii in an effort to wrap up the deal this week.
Labour's health spokesperson Annette King said people would have to pay more for their medicines, despite Mr Key's assurance they would still be subsidised.
But Mrs King said the impact would be worse than that.
"Some people are going to pay with their lives because if they extend the patent, particularly on drugs for cancer and heart disease, and we can't get access to the generic drugs for longer, then people are not going to get that access and they won't have the opportunity to extend their lives," she said.
In Parliament this afternoon, Labour Party leader Andrew Little asked whether the Prime Minister could guarantee that Pharmac would still be able to purchase life-saving drugs, when the cost increased by hundreds of millions of dollars.
Mr Key insisted the cost increases would not be that much.
"There may be a very small cost increase. If there is, the Government will fund that - but let's just test this for one second, is the Labour Party now telling us they would like to pull out of the China FTA that's had massive impacts on the New Zealand economy - no! but somehow then they could do one with the United States they think it's a bad idea."
Green Party health spokesperson Kevin Hague asked the Health Minister whether other parts of the health budget would be raided if Pharmac's costs went up.
Health Minister Jonathan Coleman told the House it wasn't like that.
"It's not a case of raiding the health budget. We've put an average of $500 million of new money into the health budget every year, it's at record levels, but more importantly... we are delivering better services than ever so the member has nothing to worry about," he said.
The TPP meeting in Hawaii is expected to wrap up on Saturday (NZT).