He said the Australian government was left with more than $50 million in legal fees after a tobacco company tried to sue it over plain packaging on cigarettes.
"Imagine that modified many times over by drug companies, agricultural chemical companies, oil companies . . . to a small country like New Zealand, they could bankrupt us with legal fees overnight."
Dr Cresswell said the trade deal could leave New Zealand like America, "where the only winners are big pharmaceutical companies".
"It's going to take two years for all the law changes and ratifications to happen."
He said that would allow a "window of opportunity" to try to persuade the Government to halt the process.
Dr Cresswell said he and the other doctors who signed the letter were feeling "frustration" and "concern regarding the privacy around all the negotiations".
He said there was an atmosphere of "grave concern" among them.
"There will be a small group of us travelling to Auckland and to Waitangi next week."
They would be "peacefully encouraging the community" with "goodness, not violence".
The doctors who signed the letter are Dr Cresswell, as well as doctors Marco Meijer, Lakshman Bandara, Tom Thompson, Matthys Gerhardus Barnard, Guy Taylor, Joanne Stephen, Jan Gregson, Derek Barrett, Matthew Wheeler, PJ Faumuina, Mohamad Firdaus bin Mohamad Hanib, John Moore, Bernd Kraus, Semisi Aiono and P J McLay.
The letter said the risks to the public included more expensive medications and biologicals, threats to public health measures such as restrictions on alcohol or harmful foods, threats to the environment, threats to sovereignty with investor-state dispute settlements being settled in foreign courts, and increased income inequality.
On Monday after being booed at Ratana over remarks on the TPP, Mr Key said the Government had the public on its side and that opponents had made up their minds about it before they saw the text or the national interest analysis.
"I think we are winning the argument actually with the New Zealand public who have seen for a long period of time that asking New Zealand businesses to compete with one hand tied behind their back in the biggest economy in the world, the United States, and the third largest like Japan, puts us in a disadvantaged position," he said at his post-Cabinet press conference.
"This is a free-trade deal that gives us access to 800 million middle-income consumers. Its economic benefits are about two-and-a-half times the size of the China free trade agreement.
"Successive governments have been trying to conclude this deal and, actually, it's great for New Zealand."
Trade ministers of the TPP countries are due to sign the agreement next Thursday. Each country has to then go through its own ratification process.
-Further story, page 4.
