A clinical trial of Zetia, a cholesterol-lowering drug prescribed to about 1 million people a week, failed to show that the drug has any medical benefits, Merck and Schering-Plough said on Monday.

The results will add to the growing concern over Zetia and Vytorin, a drug that combines Zetia with another cholesterol medicine in a single pill. About 60 percent of patients who take Zetia do so in the form of Vytorin, which combines Zetia with the cholesterol drug Zocor.

While Zetia lowers cholesterol by 15 percent to 20 percent in most patients, no trial has ever shown that it can reduce heart attacks and strokes  or even that it reduces the growth of the fatty plaques in arteries that can cause heart problems.

This trial was designed to show that Zetia could reduce the growth of those plaques. Instead, the plaques actually grew almost twice as fast in patients taking Zetia along with Zocor than in those taking Zocor alone.