GlaxoSmithKline was dealt a major blow today when scientists reported that one of its top selling drugs - Avandia for diabetes - increased the risk of heart death by 64 per cent.

Glaxo’s shares closed down more than 5 per cent or 74p at 1390p as news emerged before the market closed, even though Glaxo issued a rebuttal saying that it disagreed with the conclusions and that its own more detailed and lengthier studies had found no problems.

Avandia is Glaxo's second biggest selling drug, given approval in the US in 1999 as a medicine to reduce blood sugar in people with type 2 diabetes - the kind that arises through dietary problems - more than 60 million prescriptions have been issued since then.

Last month Glaxo reported first quarter sales of Avandia grew 19 per cent to £414 million.

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The study of Avandia, also known as rosiglitazone, was published online today by the New England Journal of Medicine .

Doctors at the Cleveland Clinic analysed 42 clinical trials of Glaxo's Avandia prescribed to more than 60 million people since it won US approval eight years ago to reduce blood sugar in people with type 2 diabetes, the kind that develops through dietary problems.

It found that there were more heartattacks among people taking Avandia than those taking a control and 39 deaths from heart problems for those on Avandia and just 22 in the control group.

Chairman of cardiovascular medicine at the clinic, the highly-respected Steven Nissen said: “The results of this analysis raise serious concerns about the cardiovascular safety of rosiglitazone,”

He said in a statement: “Unfortunately, rosiglitazone appears to increase, rather than decrease, the most serious complication of diabetes, heart disease.”

He conceded that the study was limited because it did not have access to Glaxo's original data, but added “Patients and providers should consider the potential for serious adverse cardiovascular effects of treatment with rosiglitazone for type 2 diabetes.”

He urged Glaxo to make its own research publicly available and called for more trials.

Glaxo said it had shares its data on Avandia with regulatory agencies. It pointed out that the editors of the New England Journal of Medicine had pointed out that "a few events either way might have changed the findings...the possibility that the findings were due to chance cannot be excluded."

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