The results of the study, published in The New England Journal of Medicine, were presented here Sunday night at an annual meeting of the American Heart Association.

The study has been a polarizing topic here and has also attracted the attention of a powerful senator who has been investigating the conduct of two drug makers, Merck and Schering-Plough, in relation to their sales and marketing of Zetia and a combination cholesterol drug, Vytorin, which includes Zetia. The drug makers merged this month.

The small study, with only 208 patients, has attracted outsize attention because the researchers did a head-to-head comparison of niacin and Zetia, which has been heavily marketed.

The Food and Drug Administration approved Zetia in 2002 to lower bad cholesterol, a risk factor for heart disease. But the drug has not yet proved to have a longer-term clinical benefit in reducing heart attacks and deaths. Merck, the maker of the drug, is conducting a clinical trial on that issue involving up to 18,000 patients. Statins like Lipitor have proved in studies to significantly lower the risk of heart attack.

Some cardiologists here hailed the study as an indication that the popularity of Zetia and Vytorin, which had combined sales last year of about $4.6 billion, has far outstripped their evidence of a concrete benefit on heart health. Other doctors here dismissed the study because it did not directly measure the drugs’ effects on reducing heart attacks.

Nevertheless, this study has the potential to make big waves in the use of cholesterol drugs.

“It will certainly strengthen the idea that, after you give a statin, the weight of the evidence is that, as a second agent, you should give niacin,” said Dr. Roger S. Blumenthal, a professor of medicine at the Johns Hopkins University Medical School. “That is the implication of the study.”

But Dr. Peter S. Kim, the president of Merck Research Laboratories, said Sunday in an interview that the study was limited because it did not compare the groups of patients taking a statin and a second drug to a placebo group. Furthermore, he said, a drug’s ability to improve artery-wall thickness has not been proved to automatically correlate with a reduction in heart attacks.

Zetia, he said, lowers bad cholesterol and lowering bad cholesterol is a known good.

The study results “should be compared to the overwhelming body of evidence that lowering LDL cholesterol is an important thing to do to improve cardiovascular health,” Dr. Kim said.

The study randomly assigned patients who were taking a statin and who had heart disease or a risk of heart disease to additionally take either Zetia or Niaspan.

Statins are a class of drug which lowers LDL, known as bad cholesterol because it can cause arterial thickening and lead to heart problems. The drugs work by inhibiting the production of cholesterol in the liver.

Zetia, which inhibits the absorption of cholesterol in the intestines, lowers bad cholesterol.

Niaspan is a prescription extended-release form of niacin, not the over-the-counter vitamin. Niacin increases HDL, known as “good cholesterol.” Niaspan is made by Abbott Laboratories, which financed the study.

Over the course of the 14-month study, the bad cholesterol of the patients on Zetia decreased by 19.2 percent, but the patients’ arterial wall thickness stayed the same, the study said. In the niacin group, good cholesterol increased by 18.4 percent and the carotid wall thickness decreased.

By itself, the study does not have major significance, said Dr. James H. Stein, a professor at the University of Wisconsin medical school. But taken in the context of more than 30 years of research on and use of niacin, he said, the study adds to the weight of evidence that it can be a great benefit to patients with heart disease, he said. “Compare that to Zetia where there is not a shred of evidence that it does anything good for blood vessels or heart disease,” Dr. Stein said.

On Friday, Senator Charles E. Grassley, Republican of Iowa, wrote to the Department of Health and Human Services, asking its director, Kathleen Sebelius, what action she intended to take in light of the study results. Mr. Grassley sits on the Senate Finance Committee which has jurisdiction over Medicare and its drug spending. In 2006 and 2007, the drug makers made more than $300 million through Medicare Part D in sales of Vytorin, a drug that combines Zetia and a statin, Mr. Grassley wrote.

In response to a query from a reporter, a Merck spokesman said the small trial did not change the company’s belief in the demonstrated ability of Zetia and Vytorin to reduce bad cholesterol.

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