Two popular drugs used to lower cholesterol are coming under fire.  Zetia and Vytorin, which contains Zetia, are taken by millions of Americans to lower cholesterol.  But there are major questions about how well the drugs work. 

A study in the New England Journal of Medicine compared Zetia to a much cheaper alternative drug called Niaspan. Researchers found Zetia failed to reduce plaque buildup in neck arteries. There were also more heart attacks in the Zetia group. The results caused researchers to shut down the experiment early.

"It was surprising that they found such a big difference in both the increased risks in complications and deaths among patients who are on Zetia," Dr. Jennifer Ashton with CBS News said.

But the results haven't fully convinced all doctors.

"It's hard to draw a firm conclusion about stopping Zetia use at this time. I think I will wait for more studies to support the findings that are now being made public," Cardiologist Dr. Merle Myerson said.

Merck, which makes Zetia and Vytorin, is questioning the validity of the research. The company points out the study was very small, only 363 patients. And the research was sponsored by the company that makes Niaspan - Abbott Labs.