The U.S. Food and Drug Administration (FDA) says safety data from controlled clinical trials have shown there is a potentially significant increase in the risk of heart attack and heart-related deaths in patients taking the widely used diabetes drug Avandia (rosiglitazone).

However, other published and unpublished data from long-term clinical trials of Avandia provides contradictory evidence about the risks in patients treated with Avandia, according to the FDA, which is not asking GlaxoSmithKline to take any specific action at this time.

The millions of patients taking Avandia, especially those with underlying heart disease or who are at high risk of heart attack, were encouraged to talk to talk to their doctor about this new information as they evaluate the available treatment options for their type 2 diabetes.

"FDA remains committed to assuring that doctors and patients have the latest information available to make treatment and medication use decisions," the agency said.

"In this case, FDA is carefully weighing several complex sources of data, some of which show conflicting results, related to the risk of heart attack and heart-related deaths in patients treated with Avandia," said Dr. Steven Galson, director of the FDA's Center for Drug Evaluation and Research.

"We will complete our analyses and make the results available as soon as possible. FDA will take the issue of cardiovascular risk associated with Avandia and other drugs in this class to an Advisory Committee as soon as one can be convened," he added.

The FDA has updated Avandia's labeling several times since it was approved in 1999, most recently in 2006 when the labeling was changed to include a new warning about a potential increase in heart attacks and heart-related chest pain in some individuals using Avandia.

This new warning was based on the result of a controlled clinical trial in patients with existing congestive heart failure.

Recently, Glaxo provided the FDA with a pooled analysis of 42 randomized, controlled clinical trials suggesting that patients receiving short-term (most studies were 6-months duration) treatment with Avandia may have a 30-40 percent greater risk of heart attack and other heart-related adverse events.

If confirmed, this data would be of significant concern since patients with diabetes are already at an increased risk of heart disease.