A new study raises fresh concerns about two cholesterol drugs sold by Merck and Company.

Zetia and Vytorin are drugs taken by millions of Americans to lower cholesterol, despite questions raised about how well they work.

In the study, reported in "The New England Journal of Medicine," Zetia failed to shrink buildups on artery walls. According to the report, Zetia users suffered more heart attacks.

However, the president of Merck's research lab says the number of heart attacks and other problems are too small to be conclusive in this study.

Peter Kim, the president of Merck Research Laboratories, says, "Nothing about this study should alter the prescribing habits of physicians. To the contrary, they should look to the wealth of data that points to the importance of lowering cholesterol."

Heart experts say the study is too limited for people to change their prescriptions.

Patients also should not stop taking any medicine without first checking with their doctor.