Results from the trial whose seemingly endless duration sparked a Congressional probe are finally in. The findings of the Enhance trial were far from what Merck (NYSE: MRK) and Schering-Plough (NYSE: SGP) -- the companies that market Vytorin -- were hoping for.

The good news was that Vytorin, which is a combination of Zetia and Zocor, lowered low-density lipoprotein (LDL) cholesterol -- that's the bad kind -- more than Zocor alone. That's not a big surprise, since previous clinical trials have shown that Zetia works well in combination and by itself. The previous trials are the reason Zetia and Vytorin have both become multibillion-dollar drugs.

The bad news was that the lowered cholesterol didn't translate into less plaque in the arteries. Vytorin and Zocor caused a change in thickness of the arteries that was statistically the same after two years of use.

In clinical trials, the same is often good enough, but the problem for the cholesterol-fighting duo is that Zocor is available as a much cheaper generic. Doctors may be inclined to prescribe generic statins in order to save their patients some money. That's something that competing statins such as Pfizer's (NYSE: PFE) Lipitor and AstraZeneca's (NYSE: AZN) Crestor have been trying to fight, and its one of the reasons my Foolish colleague picked Pfizer as the worst stock of 2008.

Of course, patients -- and their doctors, for that matter -- don't care about the thickness of arteries. Ultimately all they care about is whether the drugs will make them live longer. To that end, Merck and Schering are testing Vytorin in three large trials involving more than 20,000 patients, to test the drug's abilities to lower heart attacks and other cardiac events. Ultimately, Zetia's and Vytorin's futures depends on those trials.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Pfizer is a pick of the Inside Value newsletter service. The Fool has a disclosure policy.