A spokeswoman for John Key said the Government had been very clear about what New Zealand would not do in the talks.
"We are not about to negotiate away the fundamentals of New Zealand's health system or Pharmac, " she told the Otago Daily Times.
Association of Salaried Medical Specialists executive director Ian Powell said Mr Key needed to spell out what those fundamentals were.
"What is getting the Prime Minister into difficulty is the lack of transparency around these negotiations."
Pharmac was necessary to protect the interests of patient safety and financial rip-offs in negotiations with "these powerful drug monopolies".
Weakening Pharmac would increase the vulnerability of patients and taxpayers, he said.
Mr Powell's comments followed news of leaked documents from the TPP which University of Auckland law professor Jane Kelsey said confirmed fears the United States was pushing for rules on health care products which would give pharmaceutical "giants" new tools to attack agencies such as Pharmac.
The US proposals would allow drug companies to challenge every Pharmac decision as not appropriately recognising the "value" of patents, which she described as a dangerous and undefined standard.
"Adopting this standard would open floodgates of litigation against Pharmac and will ultimately raise medicine prices and ration access," she said.
Pharmac chairman, Stuart McLauchlan of Dunedin, said the organisation was apolitical and not directly involved in the negotiations, but was watching developments with interest.
He pointed out that Pharmac saved $1.3 billion every year on medications, allowing that money to be spent elsewhere in the health system.
If there were to be some trade deal the benefits would have to be more than that amount - "it's a pretty big number".
Pharmac in its present form had "certainly delivered" and was well supported across the health sector although he conceded perhaps the wider public did not fully realise its worth to the health budget.
• The TPP is a regional trade agreement between New Zealand, the US, Australia, Brunei, Chile, Malaysia, Peru, Singapore and Vietnam.
