Merck (NYSE:MRK) gaining Food and Drug Administration approval for its ezetimibe and atorvastatin combination tablet shouldn't be that difficult. Ezetimibe goes by the brand name Zetia, and atorvastatin is Pfizer's (NYSE:PFE) Lipitor. Both are approved already, have been for years.

And Merck sells Vytorin, which is a combination of Zetia and Zocor. The latter is in the same class as Lipitor. An approval should be a slam dunk.

But it hasn't been. Far from it.

In 2009, Merck couldn't get the FDA to even look at its application. The agency issued a refuse to file letter requesting additional manufacturing and stability data.

Merck resubmitted. Got its application reviewed that time. But still couldn't get the FDA to sign off on the drug. Last year's rejection requested even more data.

The pharma giant has addressed those issues (hopefully) and resubmitted the application. The FDA should make its next ruling in the first half of the year .

For doctors that are interested in lowering patients' cholesterol further, having the additional combo product should produce additional sales. Doctors are likely to be more comfortable adding a drug to patients already taking Lipitor than to switch from Lipitor to the two new drugs in Vytorin. Of course, they could always prescribe Lipitor and Zetia separately, but that requires two pills.

The problem for Merck is that the company doesn't have any outcomes data for the Lipitor-Zetia combo. It doesn't even have data proving that Vytorin is any better than Zocor alone at reducing heart issues such as heart attacks and strokes.

Results from the long-awaited trial called Improve-It that tests that hypothesis are expected later this year. If they don't come back positive, whether Merck is able to finally get the Lipitor-Zetia combo product approved won't matter that much because no one will be convinced that Zetia is doing anything useful beyond changing a laboratory result.

AstraZeneca (NYSE:AZN), whose Crestor has produced positive outcome study results, and companies such as Amarin (NASDAQ:AMRN) targeting cardiovascular disease through mechanisms other than cholesterol will likely benefit if Zetia fails to prove useful.

Fool contributor Brian Orelli has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.