The controversy surrounding Avandia deepened yesterday after a medical expert in the United States claimed that GlaxoSmithKline had tried to silence him when he questioned the drug’s safety.

John Buse, a diabetes expert from the University of North Carolina, said that he had received telephone calls from Glaxo executives as the drug went on sale in 1999, claiming that his “actions were scurrilous enough to attempt to hold me liable for a loss in market capitalisation”.

Dr Buse, who disclosed the threats of legal action in a written testimony submitted for a congressional hearing on the drug’s safety yesterday, said that later he had signed a statement drafted by Glaxo to ease concerns that his warnings had raised among shareholders.

JP Garnier, GlaxoSmithKline’s chief executive, strongly rejected the claims. “We don’t threaten people,” he said. “[Dr Buse] did issue a correction . . . There are no hard feelings.”

Dr Buse, who is expected to become the next president of the American Diabetes Association, made several attacks on Avandia after its launch in 1999. In March 2000, he wrote to the US Food and Drug Administration (FDA) giving warning that the drug might increase the risk of heart attacks. He also criticised Glaxo’s “blatant selective manipulation of data”, which he said had understated the drugs risks and overstated its benefits.

The warnings were given added weight last month when the New England Journal of Medicine published research linking the drug to cardio-vascular problems. The research prompted the US House of Representatives’ Committee on Oversight and Government Reform, the congressional chamber’s chief investigative committee, to convene a hearing into the FDA’s role in evaluating Avandia’s safety.

Dr Garnier said that Avandia risked becoming a “political football” in a broader American debate over reform of the FDA, which has been attacked for its handling of the approval of other drugs, including Vioxx. He accused the New England Journal of Medicine of “mixing politics and science”. “It’s a very bad marriage,” he said.

He also acknowledged that GlaxoSmithKline might be forced to lower its earnings guidance after a downturn in Avandia sales. He said that there were no plans to do so, but the company could “never keep [the same] guidance forever”.

Henry Waxman, the committee’s chairman, criticised Andrew von Eschenbach, the FDA’s Commissioner, for not requesting specific research into Avandia’s potential for causing heart problems after Dr Buse and, later, the World Health Organisation, gave warnings of a potential connection.

The New England Journal of Medicine research claimed that Avandia raised the risks of heart attack by 43 per cent and the risk of heart death by 64 per cent. It also questioned the FDA drug approval process. Within nine days of the report, Glaxo’s share price had fallen by 13 per cent, wiping £9 billion off its market value.

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