NEW YORK (CNNMoney.com) - Merck and Schering-Plough announced a study on Sunday showing that their combination drug Vytorin is better at lowering cholesterol than competing drug Crestor from AstraZeneca.

Vytorin reduced LDL, or "bad" cholesterol, by 52 to 61 percent, depending on the dosage, compared to reductions of 46 to 57 percent for comparable doses of Crestor, according to the study, which was conducted by independent researchers and funded by Merck and Schering-Plough.

With all dose comparisons averaged together, Vytorin lowered LDL by 56 percent, compared to 52 percent with Crestor, according to Merck (Charts) and Schering-Plough (Charts).

"This study provides ammo that we can be more effective in lowering LDL," said Dr. Michael Davidson, professor of medicine at Rush University Medical Center in Chicago, and lead investigator in the study.

The study, announced at International Symposium on Atherosclerosis in Rome, found that the competing drugs were equally beneficial in raising HDL, or "good" cholesterol, with increases of 8 percent.

Vytorin, a once-a-day pill, is a combination of two cholesterol drugs: Merck's Zocor and Schering-Plough's Zetia. The findings are important, particularly for Merck, because the company is working to offset the impending threat of a Zocor sales plunge by combining that drug with Zetia.

Zocor is one of Merck's top-selling drugs, with $4.4 billion in 2005, but its patent expires on June 23. If Merck continues to produce Zocor as a brand-name product, the price and the sales could plunge about 80 percent over the next year, because generic drug makers will produce cheaper versions, undermining the brand name price.

The Food and Drug Administration insists that generic drugs work exactly the same as their brand name counterparts. But Merck might be able to differentiate Zocor from generic simvastatin by combining its product with Zetia.

AstraZeneca (Charts) spokesman Chris Sampson said that his company revealed its own study Sunday that shows the benefits of combining Crestor with Zetia, compared to Crestor alone. Unlike Merck's head-to-head study, the AstraZeneca study did not compare Crestor to a competing drug.

In this study, patients who took the Crestor-Zetia combination reduced their LDL by an average of 70 percent, compared to 57 percent who took Crestor alone, said Sampson. Also, 94 percent of high-risk patients who took the combination achieved the medically accepted target for cholesterol levels, compared to 79 percent for those who just took Crestor.

This study was funded by AstraZeneca and conducted by independent investigator Dr. Christie Ballantyne of DeBakey Methodist Heart Center in Houston.

Merck and Schering-Plough produce and market Vytorin and Zetia through a separate company called Merck/Schering-Plough Pharmaceuticals. Vytorin and Zetia sales totaled $2.4 billion in 2005 for this company, with a tally of more than $1 billion for each product.

Crestor sales totaled $1.3 billion in 2005 for AstraZeneca. Like Vytorin, Crestor is also a once-daily pill. The drug maker, which reported $24 billion in total sales last year, has its corporate headquarter in London and its R&D headquarters in Sodertalje, Sweden.

Merck, the second-biggest drug maker in America, is based in Whitehouse Station, N.J. and reported $22 billion in 2005 sales. Schering-Plough is based in Kenilworth, N.J. and reported 2005 sales of $9.5 billion.

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