Almost exactly a year after Merck (NYSE:MRK) and Schering-Plough (NYSE:SGP) released data from their Enhance trial showing that their cholesterol-fighting drug, Vytorin, wasn't able to lower the amount of plaque in a neck artery more than Zocor did, the Food and Drug Administration says there's still enough evidence that Vytorin works to warrant keeping patients on the drug.

Thanks for the vote of confidence, FDA, but I think the damage has already been done.

Year-over-year sales of the duo's cholesterol-fighting drugs, Zetia and Vytorin (the latter being a comination of Zetia and Zocor), fell by 15% in the third quarter of last year, as patients migrated to Pfizer's (NYSE:PFE) Lipitor, AstraZeneca's (NYSE:AZN) Crestor, or generic versions of statins such as Merck's Zocor or Bristol-Myers Squibb's (NYSE:BMY) Pravachol.

In citing why Vytorin should still be used, the FDA points out that the drug still lowers "bad" LDL cholesterol more than Zocor does, and the surrogate marker is a good predictor of risk for cardiovascular disease.

That seems to be a change in stance from when the agency didn't accept LDL levels, instead forcing clinical trials to measure outcomes such as heart attacks for drugs such as Merck's MK-0524A and Isis Pharmaceuticals' (NASDAQ:ISIS) and Genzyme's (NASDAQ:GENZ) mipomersen. Although it's possible that the agency will now be more lenient with those drugs, investors probably shouldn't count on it. I think the divergence is more likely to be a difference of opinion between agency departments, and that the staffers handling approvals will have the last word on MK-0524A and mipomersen, not the department reviewing post-approval trials.

Ultimately, the level of sales of Vytorin and Zetia will be determined by the results of Improve-It, a trial that measures Vytorin's effect on cardiac events such as heart attacks. Unfortunately, the trial's results aren't due until 2012, so sales will likely be relatively stagnant until then, despite the FDAs green light for the drug.

Further pharmaceutical Foolishness:

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Pfizer is also an Inside Value pick. The Fool owns shares of Pfizer. Check out the Fool's disclosure policy.