Prime Minister John Key expects the Trans-Pacific Partnership (TPP) to be signed as negotiated, despite reports the United States will demand changes on certain next-generation drugs.
Opponents of the multinational trade deal are warning the US will try to strong-arm other nations into accepting longer monopoly protection periods for drugs known as biologics.
Representatives from the 12 nations involved in the TPP are due to sign the agreement in Auckland on Thursday.
The director of US-based advocacy group Public Citizen Global Trade Watch, Lori Wallach, said US Trade Representative Michael Froman - under pressure from Republicans - would seek eight years' monopoly protection rather than five for biologics.
"Mike Froman has already said publicly that he will be asking the other countries to make the additional 'clarification' as he's calling it but in fact it's concessions - that it's eight years, not the five years in the text, for biologics exclusivity."
Peru had already buckled, Ms Wallach said.
"That was one of the partners of New Zealand in insisting that exclusivity not go past five years. The [Peruvian] Ambassador... got ticked off, in the box that Peru is signed off now, by giving a speech in Washington saying publicly Peru recognises there is eight years' exclusivity for biologics."
Mr Key told Morning Report he had not seen details on US demands for new concessions, but expected the deal to be signed "as per the text" of the agreement.
"My understanding though is that when all of the negotiations around biologics were done before the agreement was signed, we got advice from our health officials about the impact of that - and the cost effects were nil on New Zealand.
"The reason for that is because we follow best practice.
"We've got an agreement, we intend to stick to the agreement, and the advice we have on the agreement we've got is that the impact on New Zealand's pharmaceutical costs is essentially zero."
Five years' protection sufficient - McClay
Analysts have said the TPP deliberately fudged the protection period for biologics to ensure agreement was reached.
Last year, health economists estimated a three-year extension could add up to $75-150 million to New Zealand's health bill - with that calculation based on just seven biologics.
But New Zealand's Trade Minister, Todd McClay, said the protection period was five years and he had no interest in looking at it again.
"I've had no approach from the US or any other delegation over this issue. I have had discussions more generally around some of the conditions in [the] TPP with my colleague in Australia, and we're united in our view that the agreement that was reached over five years' protection is sufficient. It was part of a wider deal, and New Zealand has no interest in looking at it again."
International Business Forum executive director Stephen Jacobi said US politicians wanted more favourable terms but it was not going to happen, as every country had made compromises.
"I think it will be very difficult to start a conversation about trying to water down some of the things that have already been agreed," he said.
"This has been a very difficult and lengthy negotiation. The consensus around that area was very hard to get. I can't imagine that Ambassador Froman's colleagues will be willing to entertain that sort of representation."
The deal's opponents, however, did not expect that to be the end of it - saying the US would continue to put pressure on other countries to accept concessions and argue the deal had no chance of getting through Congress without them.