Labour and the Greens are calling for government assurances that Pharmac won't be a pawn in the Trans Pacific Partnership (TPP) negotiations.
Prime Minister John Key and Trade Minister Tim Groser have both previously said it won't be damaged, but they won't discuss details of the negotiations around it.
The Greens say three United States-sponsored documents leaked over the weekend show there would be a negative impact on the Government's drug-buying agency if proposals in them were adopted.
"The US proposals would open up Pharmac's decision making to litigation from United States patent attorneys and it would extend the life of drug patents, restricting Pharmac's ability to use cheaper generics," Green's co-leader Russel Norman said on Tuesday.
Labour's trade spokeswoman Maryan Street says protecting Pharmac is one of her party's bottom lines.
"The Labour Party supports moving forward with the TPP but does not support signing it at all costs," she said.
"Retaining our effective drug-buying agency, which is envied by many countries, is essential."
Pharmac has been credited with lowering the cost of drugs and making them available to more people.
The TPP negotiations are about extending the existing free trade agreement between Brunei, Chile, New Zealand and Singapore to include Australia, the US, Malaysia, Peru and Vietnam.
NZN