Trade Minister Andrew Robb flew to Atlanta on Monday to continue negotiations on the Trans-Pacific Partnership agreement with officials from 12 nations, including the United States, Japan and Singapore following last month's failed discussions. One of Australia's major sticking points was its refusal to buckle under pressure from the United States to extend the exclusivity period for biologic drugs from five to 12 years.

Medical patents for biologic drugs - including treatments for cancers and diseases such as rheumatoid arthritis - last 20 years in Australia. Pharmaceutical companies must currently wait an extra five years to access the clinical data needed to create similar and cheaper versions, known as biosimilars.

The United States trade representative is now seeking an eight-year exclusivity period, but Mr Robb has pledged not to budge beyond Australia's "red line" of five years. Any changes would also require changing Australia's Therapeutic Goods Act, which would struggle to pass the Senate.

But international trade agreement experts say a new proposal is being floated among US pharmaceutical companies, which, if accepted, would mean biosimilars would not be able to be registered and available for consumers for three more years while they underwent safety tests.

It is believed Australian negotiators are seriously considering such an administrative compromise ahead of the talks, which Mr Robb has said could make or break the deal.

Co-ordinator of the Australian Fair Trade and Investment Network, Dr Patricia Ranald, said safety checks were already required for biosimilars in Australia: "Even if the negotiators come up with a different way to achieve market exclusivity on biologic medicines it would still have the same effect of [keeping] biosimilars out of the market," she said.

"The other nations don't have to agree, but Australia and others have been holding out so if Australia caves in, then it will weaken other countries' resistance," Dr Ranald, an expert on trade agreements and politics from the University of Sydney, said.

Trade negotiations are done in secret, with the final text of the agreement not to be publicised until after it is reached.

"Our health policy shouldn't be changed every two years through trade negotiations behind closed doors," Dr Ranald said. "It should be democratically decided in our normal parliamentary process."

La Trobe University public health researcher Dr Deborah Gleeson said there was a risk the Pharmaceutical Benefits Scheme would pass on rising costs from pharmaceutical monopoly protections to consumers. Research showed that patients, particularly disadvantaged groups, skipped doses because they could not afford rising medical costs, which "translates to avoidable hospitalisation and morbidity".

Dr Gleeson has warned that pharmaceutical monopoly protections already cost the Australian health system hundreds of millions of dollars a year.

Mr Robb has previously dismissed such concerns, saying: "The government will not support outcomes that would increase the prices of medicines for Australians or adversely affect our health system more generally; end of story."

Other countries included in the proposed trade deal are New Zealand, Malaysia, Brunei, Canada, Chile, Mexico, Peru and Vietnam.