The US Food and Drug Administration was guilty of "profound regulatory failure" in its oversight of the painkiller Vioxx, an FDA scientist has claimed.
Vioxx has been prescribed to twenty million patients since 1999
Dr David Graham told a Senate inquiry that he felt pressured to water down findings from a study linking the drug to greater chances of heart attacks.
The chief executive of Merck & Co, the maker of Vioxx, told the hearing the firm had acted properly over the drug.
Merck pulled Vioxx after a study showed it doubled the risk of cardiac arrest.
Vioxx was being used by two million people worldwide at the time it was withdrawn on 30 September. Sales of the drug, which was prescribed to combat arthritis pain, were worth £2.5bn (£1.3bn) to Merck in 2003.
A lawsuit has been filed in the US alleging that the company misled users about the dangers involved in taking the drug.
Dr Graham, associate director for science in the FDA's Office of Drug Safety, claimed he had felt under pressure from supervisors to downplay the findings of a study of patients' insurance records indicating that Vioxx users had a 50% higher chance of a heart attack and sudden cardiac death than those using a rival medicine, Celebrex.
"I would agree the FDA as currently configured is incapable of protecting America against another Vioxx," he told the Senate Finance Committee.
The Committee is looking into the background to the drug's withdrawal and its effect on patient safety.
On Wednesday, the FDA's acting commissioner Lester Crawford said Dr Graham had violated procedure by submitting research to a medical journal without the agency's approval.
Raymond Gilmartin, Merck's chief executive, told the hearing that the firm believed wholeheartedly in Vioxx and had followed rigorous scientific procedures every step of the way regarding the drug.
"Over the past six years, we have promptly disclosed results of numerous Merck-sponsored studies to the FDA, physicians, the scientific community and the media," Mr Gilmartin said.
He added: "My wife was taking Vioxx, using Vioxx, up until the day we withdrew it from the market."
Senator Charles Grassley, chairman of the committee, said he was concerned that the FDA had a far "too cosy" relationship with pharmaceutical companies.
"Now we have scientists in this particular case who are being harassed within the agency because of sticking to their own science," Mr Grassley said.
Merck's share price, which plunged to an eight year low after the withdrawal of Vioxx, registered little reaction to the hearings, closing slightly higher.