FDA warns GSK about failure to disclose Avandia data. Source: ABC News

The FDA reported Tuesday that GlaxoSmithKline (GSK) failed to report clinical data on its diabetes drug Avandia to U.S. regulators.

In a letter posted on the FDA's website, the agency criticizes GSK for not including post-marketing information about Avandia in reports to the FDA.

“The specific violations noted in this letter are serious and may be symptomatic of underlying post-marketing safety reporting failures," according to the letter. The FDA has instructed GSK to correct the violations or risk “regulatory action without further notice.”

Scrutiny of Avandia has been high and sales have been down since May 2007, when a report in the New England Journal of Medicine linked the treatment to an increased risk of a heart attack.

The agency said it discovered the data in an inspection last year at GSK's site in Research Triangle Park, N.C., where the London-based pharma company employs 5,000.

“We take these findings seriously, and corrective steps to make sure we file periodic reports completely and promptly have been taken or are underway," Ronald Krall, GSK's chief medical officer.

Avandia was the world's top-selling diabetes drug until sales plunged in May 2007, after the drug's possible health risks became public. U.S. revenue from Avandia and related projects dropped 55 percent in the fourth quarter of 2007.