Zetia, a cholesterol-lowering drug prescribed to about 1 million people each week, has no medical benefits, according to a trial by Merck and Schering-Plough.

While the pill does lower cholesterol by 15 percent to 20 percent, trials have not shown that Zetia reduces heart attacks or strokes, or that it reduces plaques in arteries that can lead to heart problems.

The current trial, which studied whether Zetia could reduce the growth of plaques, found that plaques grew nearly twice as fast in patients taking Zetia along with Zocor than in those taking Zocor alone.

Patients who took both Zetia and Zocor received it in the form of Vytorin, a pill that combines the medications.

Experts have called the results “shocking,” saying that Zetia should not be prescribed unless all other cholesterol drugs have failed.

The results also add to the controversy over Merck and Schering-Plough’s delays in releasing them. The trial was completed in April 2006, with results scheduled to be released in March 2007. However, the companies missed several deadlines, and only agreed to release the results after media outlets focused on their continued delays.

Zetia and Vytorin account for about 20 percent of the cholesterol drugs on the U.S. market.