An emergency physician at Whanganui Hospital, Dr Cresswell said one concern was that the agreement would undermine Pharmac and increase the cost of pharmaceutical drugs for New Zealanders.
"Drug companies would be able to challenge every decision made by Pharmac," he said.
He said Australia had faced a $50million legal challenge when it tried to introduce plain packaging on cigarettes.
"I think people are starting to wake up to the implications of this and seeing it's not just a radical element opposed to this. It's sensible people."
Dr Cresswell said more protests were planned throughout the day.
Mr English is due to speak at a Chamber of Commerce lunch in Wanganui this afternoon.
About six police officers were present at this morning's protest. "Everyone had the right to protest as long as they are doing it in a safe manner that's not disrupting the public,' sergeant Andy Reid said.
"As long as they're not breaking any rules and causing a problem, police will be leaving them alone."