Rosiglitazone, a compound found in two of GlaxoSmithKline’s products to treat type 2 diabetes, can sometimes cause a side effect in the eye known as macular edema, the company has warned.

Post-marketing surveillance has revealed a few ‘very rare’ cases in which macular edema has either occurred or worsened after patients have taken Avandia (rosiglitazone) or Avandamet (rosiglitazone plus metformin), and in one case resolved after treatment with the drug was discontinued, said GSK in a letter to doctors. Macular edema is known to occur in tandem with diabetic retinopathy, a well recognised complication of diabetes.

The majority of patients in whom the side effect developed also reported peripheral edema, or swelling of the legs, ankles and feet, the company said.

GSK has updated the labelling for Avandia and Avandamet to include a warning of the potential side effect in the ‘Precautions’ section of the products’ datasheets and patient information leaflets. It will also be included on Avandaryl (rosiglitazone and glimepiride), a new product for diabetes which was approved by the US Food and Drug Administration towards the end of November 2005 and is due for launch before the end of the month.

The side effect had already been flagged up by the European Medicines Agency and Health Canada last month, and seems to occur so rarely that it is unlikely to have any significant impact on sales of GSK’s rosiglitazone-based products, according to analysts.

GSK estimates that it affects one in 10,000 patients receiving rosiglitazone. All told, more than six million people have been treated with the drug.

Combined turnover of Avandia and Avandamet jumped 22% to £355 million in the third quarter of 2005, with growth of 21% to £265 million in the USA and soaring 44% to £40 million in Europe.