Both men have faced criticism from those who call them zealots. The F.D.A. is about to examine data suggesting that Avandia might not be so dangerous after all. Some critics say Dr. Butler overstates his conclusions and that his findings have not been replicated by others.

“Basically, no one in the entire world over the last 10 years, with thousands of animals,” has found what Dr. Butler found, said Dr. Daniel J. Drucker, a professor of medicine at the University of Toronto and a consultant to many drug companies.

Still, Dr. Butler is not easy to write off. He is a former editor of Diabetes, the flagship journal of the American Diabetes Association. And he has some defenders.

“He should be an American hero, actually, a rugged individualist who is not going to be browbeaten,” said Dr. Edwin Gale, professor emeritus at the University of Bristol in Britain, who recently wrote a commentary with Dr. Butler on the drugs.

Dr. Butler was born in Kenya to British parents, though he has worked in the United States since 1987 and is an American citizen. His wife, Dr. Alexandra E. Butler, a pathologist who occupies the office next to his, has also worked on some of the studies.

In the last month, lawyers defending drug companies against a lawsuit claiming that Byetta had caused a patient’s pancreatitis, subpoenaed virtually all of Dr. Butler’s records.

“I think the message here is they want him out of business,” said Brian Depew, a lawyer representing the plaintiff, Ross Hubert of New Hampshire, who claims that Byetta caused him to get pancreatitis. Dr. Butler said U.C.L.A. told him not to comment on the subpoena.