Florida ? 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.

The results 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.