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GlaxoSmithKline, Europe's biggest drug-maker, is racing to shore up confidence in one of its most important medicines after a study in a respected US journal suggested that it dramatically increased the risks of having a heart attack.
The drug in question, Avandia, is taken by millions of Americans and others around the world who suffer from diabetes.
It contributed £414 million ($1.1 billion) to GSK's sales in the first three months of this year.
A study published this week by the New England Journal of Medicine (NEJM) analysed all the previously published data from studies of Avandia and calculated that taking the drug increases the chance of having a heart attack by 43 per cent.
The increase in the risk of having a fatal heart attack was even higher.
GSK immediately questioned the methodology of the study, which was conducted by Steven Nissen, a cardiologist at the respected Cleveland Clinic in Ohio - but more than US$10 billion ($13.7 billion) was wiped off the value of the company in afternoon trading in the US in the wake of the study.
A GSK statement said the study was based on incomplete evidence and a methodology that the author admits has significant limitations.
The company said: "The NEJM paper is based on an analysis of summary information that combines a number of studies, which is not the most rigorous way to reach definite conclusions about adverse events.
"Each study is designed differently and looks at unique questions: for example, individual studies vary in size and length, in the type of patients who participated, and in the outcomes they investigate.
"The data compiled from these varied studies is complex and can be conflicting."
A spokesman for GSK said that it was conducting numerous long-term studies of Avandia, all of them being overseen by independent safety monitors. None of the monitors had flagged any safety concerns, he said.
Avandia is used to treat Type 2 diabetes, the most common form of the disease, which is linked to obesity. Avandia helps sensitise the body to insulin and was considered a breakthrough for blood-sugar control.
GSK also sells pills that combine the active ingredients in Avandia with other drugs, marketing them under the brands Avandamet and Avandaryl.
"Unfortunately, [Avandia] appears to increase, rather than decrease, the most serious complication of diabetes, heart disease," Nissen wrote.
"Unless this can be refuted, which I rather doubt, then this is going to seriously damage one of the cornerstones of Glaxo going forward," said Paul Diggle, an industry analyst at Nomura Code Securities in London.
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