Updated

The controversy surrounding GlaxoSmithKline's diabetes drug Avandia mounted Wednesday as a medical expert prepared to tell lawmakers executives threatened him with legal action when he first raised questions about the treatment's safety.

Dr. John Buse was contacted by SmithKline Beecham in 1999 after drawing attention to a trend in heart problems among patients using Avandia, according to testimony he was prepared to give a House committee. Buse says the representatives from SmithKline, which combined with GlaxoWellcome in 2000, mentioned that some in the company wanted to hold him accountable for hurting sales of the drug.

Buse, soon to become president of the American Diabetes Association, said he eventually signed a clarifying statement with the company that was used to ease concerns from investors.

One year later, Buse sent a letter to the Food and Drug Administration raising the same concerns.

Glaxo's head of research, Moncef Slaoui, and the head of the FDA will also testify before the House Committee on Oversight and Government Reform.

Rep. Henry Waxman, D-Calif., a frequent critic of FDA and the chair of the oversight committee, announced the hearing after an analysis in the New England Journal of Medicine concluded Avandia could raise patients' risk of heart attack by more than 40 percent.

In his opening statement, Waxman said FDA officials failed to act on warnings from Buse and others in the eight years since Avandia was approved.

"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 is among a group of House Democrats who want to give FDA more powers to regulate drug companies. The Senate recently passed a version of a bill that makes major changes to FDA's drug safety system, though companion legislation has not yet appeared in the House.