Every large-cap pharma seems to have at least one marketed drug that produces endless streams of woe for them. In GlaxoSmithKline's (NYSE: GSK) case, the drug in question is its type 2 diabetes compound, Avandia. Yesterday, Glaxo announced that the FDA had sent it a warning letter regarding its failure to meet certain post-marketing requirements for Avandia.

Since last year, when a meta-analysis came out in the "New England Journal of Medicine" that showed Avandia may be associated with an increase in cardiovascular-related adverse events, sales of the drug have taken a nosedive. In its 2007 fourth-quarter financial report, for instance, worldwide sales of Avandia, and Glaxo's combo drugs with Avandia, were down 43% year over year, to $472 million.

The FDA had expected Glaxo to report periodically on the results and progress of all post-marketing safety studies for Avandia. European Union medical authorities requested safety studies of their own for Avandia, and Glaxo failed to report the status of these studies to the FDA.

Glaxo thought that because these studies were initiated at the behest of the European Medicines Agency, it didn't have to report them. The FDA argued it didn't matter who requested the additional studies and, because they were related to the drug's safety, they fell under the agency's mandatory post-marketing reporting requirements for Avandia.

Is all this a big deal? Possibly. The FDA did sound annoyed in the warning letter to Glaxo. The agency requested that Glaxo review its post-marketing reporting for every single one of its approved therapies, and until its post-marketing reporting violations are corrected, "any pending [drug marketing applications] submitted by [GlaxoSmithKline] may not be approved."

That part about potential delays with compounds awaiting approval is why shares of POZEN (Nasdaq: POZN) were down yesterday. It is awaiting an FDA review on its migraine drug that is partnered with Glaxo and has an FDA decision expected in the middle of this month.

One has to wonder what possible effects the FDA's warning could have on other Glaxo-partnered compounds under review, like Adolor's (Nasdaq: ADLR) potential post-operative ileus compound, Entereg. Glaxo was responsible for a good chunk of that drug's development. Neither of these compounds' marketing applications is in Glaxo's name, so perhaps any FDA anger at Glaxo will have little collateral damage.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has an A-plus disclosure policy.