The Food and Drug Administration will require tougher warnings about heart failure on the diabetes drugs Avandia and Actos, FDA Commissioner Andrew von Eschenbach said at a congressional hearing Wednesday.

Von Eschenbach revealed that the FDA is ordering GlaxoSmithKline to add a "black box" warning to Avandia and ordering Takeda Pharmaceuticals to do the same for its competing diabetes drug Actos, strengthening existing warnings about a condition in which the heart does not adequately pump blood. The issue is separate from an analysis in the New England Journal of Medicine that said Avandia increased the risk of heart attack.

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The concerns about Avandia prompted some Democratic lawmakers to rebuke the Food and Drug Administration and call for increased regulation of the pharmaceutical industry.

The controversy intensified when a medical expert told the House Committee on Oversight and Government Reform that executives threatened to sue him in 1999 when he first drew attention to heart attack risks linked to Avandia.

Rep. Henry Waxman, D-Calif., who chairs the committee, lambasted the FDA for not alerting consumers sooner about the drug's potential dangers.

"Despite additional warnings from outside experts, despite the millions of patients who rely on Avandia to control their blood sugar, and despite the potential risks involved, FDA never required the manufacturer to conduct a thorough post-market study of its heart risks," Waxman said.

Waxman called Wednesday's hearing after the New England Journal of Medicine's analysis released last month concluded Avandia could raise patients' risk of heart attack by more than 40 percent.

Von Eschenbach told Congress his agency is still reviewing dozens of often contradictory studies and analyses on the drug's effects on diabetics. FDA has scheduled a July 30 meeting to have a group of outside experts look at the data.

But he caught Glaxo executives by surprise when he revealed news of the labeling change, though the company said it would comply. The tougher warnings, outlined in a so-called "black box," are the most serious a drug label can bear.

"It wasn't our understanding that they had come to a final conclusion on this issue, but if they have then obviously we'll make the change," said Chris Viehbacher, president of Glaxo's U.S. pharmaceuticals business.

The British drug maker argues that Avandia is safe and that its own patient studies are the most reliable measure of the drug's safety, although outside experts say the company's results are inconclusive.

Dr. John Buse, who is soon to become president of the American Diabetes Association, told lawmakers that after he drew attention in 1999 to heart problems among some patients using Avandia, SmithKline Beecham, which later combined with GlaxoWellcome, warned him that some executives wanted to hold him accountable for a $4 billion drop in the company's stock.

In a letter to SmithKline distributed at the hearing, Buse wrote: "Please call off the dogs. I cannot remain civilized much longer under this kind of heat."

Buse, who is head of endocrinology at the University of North Carolina, said he eventually signed a clarifying statement with the company that was used to ease concerns from investors. But one year later, Buse sent a letter to the FDA raising the same concerns.

Glaxo said in a statement: "We regret if, at any time, Dr. Buse felt the conduct of any employee was contrary to the spirit of open, scientific debate."

More than 6 million people worldwide have taken Avandia or a related drug, Avandamet, since it came on the market in 1999. Glaxo reported total U.S. sales of $2.2 billion for the drug last year.

For its part, Glaxo released the most recent results from a large-scale study of Avandia ahead of the hearing, saying the data show the drug is no more dangerous than other leading diabetes drugs.

The Senate recently passed a version of a bill that makes major changes to FDA's drug safety system. Companion legislation has not yet appeared in the House, though Waxman and other Democrats said they favor giving FDA more power to require companies to conduct drug safety studies.

Shares of GlaxoSmithKline PLC fell 44 cents to $51.46 Wednesday.