(CBS) - 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.

Merck, which makes Zetia and Vytorin, says it stands by the safety of the drugs. It is questioning the research, pointing out the study was very small in including just over 200 patients.

Also, they point out the research was sponsored by the company that makes Niaspan, Abbott Labs.

Cardiologists are now weighing all the information.

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

But it's believed the latest research will cause some doctors to steer their patients away from Zetia and Vytorin.