It's a lot easier to prove that something could happen than that it does happen. That's the problem that Merck (NYSE:MRK) has run into with its cholesterol drugs Zetia and Vytorin, the latter of which is a mixture of Zetia and cholesterol drug Zocor.

In last year's SEAS trial, patients taking Vytorin got cancer at a higher frequency than those in the placebo group, but the numbers were small -- 93 cancer cases for the Vytorin group and 65 for those taking a placebo. So the Food and Drug Administration pored over data from other trials, including a couple of large 20,000-subject trials that haven't been concluded, to see if it could determine if the increased rate was an anomaly.

Yesterday the FDA said that it's "unlikely" that Vytorin or Zetia increase the risk of cancer, but added that it "cannot be definitively ruled out." That's a lot of ambiguity for an industry that thrives on binary events.

Still, the news is really the best that Merck's investors could have hoped for. The FDA will probably need years of additional clinical data to definitively rule out an increased risk of cancer. It took about 10 years for the FDA to rule out a cancer risk for statins like Merck's Zocor or Bristol-Myers Squibb's (NYSE:BMY) Pravachol.

Unfortunately, the damage is already done. Not so much by the cancer story, but by the lack of efficacy demonstrated by SEAS and an earlier trial, Enhance, which failed to show that Vytorin and Zocor decrease plaque buildup in patients' arteries. While other cholesterol drugs like Pfizer's (NYSE:PFE) Lipitor, AstraZeneca's (NYSE:AZN) Crestor and Abbott Labs' (NYSE:ABT) Niaspan have held their own or increased, sales of Vytorin and Zetia dropped 12% year over year in 2008 and an additional 14% so far this year.

There's still potential to get back to the insane 30%-plus growth rate that Vytorin had before Enhance and SEAS. Data from two large trials -- SHARP and IMPROVE-IT -- measuring cardiovascular events like heart attacks and strokes are expected in 2010 and 2012, respectively. That's the type of measurement that patients and their doctors really care about, not whether it decreases the level of plaque in arteries.

Until then, investors will just have to wait.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool has a disclosure policy.