The possible side-effects of one of GlaxoSmithKline’s biggest drugs fuelled further concern yesterday after a new clinical study suggested that the group’s blockbuster diabetes treatment could lead to bone fractures in men as well as women.

Avandia, GSK’s type 2 diabetes drug, is the pharmaceutical group’s second-biggest-selling drug, with sales of £1.6 billion last year.

Yesterday Diabetes Care, the American medical research journal, published an article which claimed that men taking Avandia may be exposed to similar risks of suffering bone fractures as have previously been reported in women taking the drug.

GSK sought to dismiss the report, pointing out that the study had involved an examination of only 32 male patients taking the drug and was a retrospective study, in which researchers trawl through old data and seek to find statistical trends, rather than examining new patients.

In May, a critical medical study linked Avandia with an increased risk of heart attack. The findings sent prescriptions for the drug plunging and triggered a fall in GSK shares, which are near a 2½-year low.

Last year, another article suggested that Avandia was associated with increased incidence of bone fractures in women.

Yesterday, Mike Ward, an analyst for NomuraCode, said: “The overall concerns about Avandia and its side-effects are significant, as witnessed by the company’s share-price fall. Avandia is the central issue for investors right now. It has been a difficult time for Glaxo. We have to factor in that there is a possiblity of litigation.”

Mr Ward said that after claims of increased heart attack risks were published, the bank reduced its 2011 revenue forecast for Avandia from £3 billion to £1.9 billion.

GSK published its own study on the side-effects of Avandia, comparing them with the similar type 2 diabetes drugs Metformin and Glyburide. It found that the number of fractures suffered by male patients to be “comparable among the three drugs”.

The company has since used the findings from its trials to incorporate information about possible side-effects when selling the drug, so that both doctors and patients can read about risks associated with the treatment.

However, GSK will face questioning at the end of this month from an advisory committee of the US Food and Drug Administration (FDA) when it meets to discuss diabetes treatments and risks associated with Avandia.

Mr Ward suggested that the FDA may suggest that GSK publish more data with the drug on side-effects, known as “label tightening”. He said that there was also a possibility, albeit an unlikely one, that the FDA could suspend the drug.

A GSK spokesman said: “The conclusions of this study are limited and their clinical relevance is unknown. The study is a retrospective study of bone density involving very low patient numbers. It does not correct for potential confounding factors, such as age, other medical conditions, duration of diabetes, and medications that could affect bone density.”

GlaxoSmithKline shares closed up 12p at £13.03.

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