Opposition parties have called on Prime Minister John Key to rule out weakening Pharmac after leaked trade documents showed the United States wanting greater access and price controls on pharmaceuticals.
Pharmaceuticals are one of the potential stumbling blocks in the Trans Pacific Partnership talks in Lima, where negotiators are working to form broad outlines of an agreement before the Apec leaders' meeting in Honolulu in mid-November.
The TPP is a regional trade grouping of Australia, the United States, Chile, Malaysia, Peru, Singapore, Vietnam and New Zealand.
Critics claim the leaked US texts show it wants terms that would allow large US pharmaceutical companies to "wage a war of harassment against Pharmac", opening it to costly litigation over its decisions.
Labour and the Green Party have demanded the Government rule Pharmac out of the negotiations.