What's the only thing more embarrassing for a drugmaker than getting a "complete response letter" for a New Drug Application (NDA) from the Food and Drug Administration? Having the FDA refuse to file its NDA in the first place.

A "complete response letter" is sent after a review is complete and the FDA needs additional information before a drug can be approved. When the FDA refuses to file an NDA, the agency is essentially saying that the drugmaker doesn't have it together enough to get the application filled out correctly.

It usually happens to small drugmakers with less experience about what the agency wants. Acorda Therapeutics (NASDAQ:ACOR), Cardiome Pharma (NASDAQ:CRME), and Pharmacyclics, have all had their drug applications sent back after nothing more than a cursory review for completeness by the FDA.

But for it to happen to Merck (NYSE:MRK), which has plenty of experience submitting NDAs over its long history, is really embarrassing. So embarrassing that the company snuck the announcement in its quarterly 10-Q, only saying that the FDA wants "additional manufacturing and stability data" for MK-0653C. Think Merck's regulatory department might be a little distracted, worrying about job cuts after the merger with Schering-Plough (NYSE:SGP)?

The funny thing is, there seems to be little reason to rush MK-0653C's approval. The drug is a combination of Merck's (NYSE:MRK) Zetia and Pfizer's (NYSE:PFE) Lipitor. Combining cholesterol-lowering drugs from different classes is fairly common. Merck's Vytorin is a combination of its Zetia and Zocor, and Abbott Labs (NYSE:ABT) and AstraZeneca (NYSE:AZN) are working on a combination of their TriLipix and Crestor, respectively. But, as far as I know, Merck doesn't have a partnership with Pfizer, and it would probably have to wait until the patents expire in 2011 before selling the combination drug.

Then again, considering the glacial speed at which the FDA is moving these days, maybe it's never too early to start the drug approval process.

Is this a one-time slipup or a sign of bad things to come for Merck? Let us know what you think in Motley Fool CAPS. Make an out- or underperform call on these companies or post a pitch about where you think Merck is going. It's free, fun, and Foolish.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Pfizer is a recommendation of the Inside Value newsletter. The Fool's disclosure policy dots its t's and crosses its i's. Wait. Reverse that.