ORLANDO, Fla. — A new study raises fresh concerns about Zetia and its cousin, Vytorin — drugs still taken by millions of Americans to lower cholesterol, despite questions raised last year about how well they work.

In the study, Zetia failed to shrink buildups in artery walls, while a rival drug, Niaspan, did so significantly. Zetia users also suffered more heart attacks and other problems, although the numbers of these events were too small to draw firm conclusions.

Zetia "has been on the market for about seven years and we still haven't proven that it improves clinical outcomes," said Dr. Roger Blumenthal, preventive cardiology chief at Johns Hopkins University. The new results will be "very influential" in getting more doctors to turn to Niaspan, he said.

He wrote an editorial accompanying the results, which were presented Sunday at an American Heart Association conference and published on the Internet by the New England Journal of Medicine.

The study is too limited to warrant changing practice, heart experts said. Patients also should not stop taking any heart medicine without checking with their doctors, they warn.

Statins such as Lipitor and Crestor have long been used to lower LDL, or bad cholesterol, and are known to cut the risk of heart problems. Nevertheless, many statin users still suffer heart attacks, so doctors have been testing adding a second medicine to further lower risk.

One they are trying is Niaspan, a slow-release version of niacin, a type of B vitamin that raises HDL, or good cholesterol. Another is Zetia, which lowers bad cholesterol in a different way than statins do, by blocking its absorption in the gut.

Vytorin is a pill that combines Zetia with a statin. Both are sold by Merck & Co. of Whitehouse Station, N.J. Niaspan is made by North Chicago, Ill.-based Abbott Laboratories. All three of these drugs cost between $3 and $4 a day, though niacin has been sold as a cheap generic for decades. The heart association advises consumers against using over-the-counter vitamin versions in place of prescriptions, though, because they may have purity and potency issues and other risks.

The new study was sponsored by Abbott, and several study leaders have been paid speakers or consultants to the company or to rival drugmakers.

Researchers enrolled 363 people with heart disease or a high risk for it who had been taking statins for six years on average. Many were from Walter Reed Army Medical Center, where study leader Dr. Allen Taylor formerly worked.