The risks and benefits of a best-selling anti-cholesterol drug remain thoroughly muddled despite a new analysis that purports to exonerate it from the worst safety concerns. For now, patients and doctors are best advised to use Vytorin only as a last resort when other treatments have failed.

A huge marketing effort propelled Vytorin and Zetia to blockbuster status. Then the bad news started coming in. A small clinical trial indicated that while Vytorin reduced cholesterol levels, it failed to slow the growth of fatty plaques in the arteries. A second trial showed that Vytorin did not help people with heart-valve disease avoid further heart problems. Worse yet, that trial suggested that Vytorin increased the risk of developing and dying from cancer, an unexpected and frightening finding that seemed implausible to many scientists.

Now the New England Journal of Medicine has published a broader analysis led by researchers at Oxford University that combined the scary study with interim results from two larger clinical trials still in progress. The overall conclusion was reassuring. Vytorin did not seem to increase the risk of getting cancer, although there were hints that it might increase the risk of dying from cancer once you got it. The analysts suggested that this finding might be a fluke. The journal’s editors warn that the risk can’t be dismissed until more data comes in.

Experts now hope that the ongoing clinical trials will yield clearer results as the passage of time gives more opportunity for cancers to develop. Unfortunately, the trials may not be completed until 2012 at the earliest.

The F.D.A. will conduct its own analysis of the cancer risks. The agency also needs to evaluate whether it should demand larger and more definitive clinical trials before approving certain drugs. Meanwhile, millions of people are still taking Vytorin and Zetia without knowing whether they are safe and clinically effective.