CHICAGO

Two widely prescribed cholesterol-lowering drugs, Vytorin and Zetia, may not work and should be used only as a last resort, a panel of four cardiologists told an audience of more than 5,000 people at a major cardiology conference on Sunday.

Instead, doctors and patients should rely more heavily on older cholesterol-lowering drugs called statins, which have proven benefits and can be cheaper, the panel said.

''The strongest recommendation we can make on this panel is to go back to statins,'' said Dr. Harlan M. Krumholz, a cardiologist at Yale. ''They work.''

Statins include drugs like Lipitor and simvastatin, the generic version of Zocor. But other, lesser-known drugs like the vitamin niacin should also be tried before Vytorin and Zetia, the panel said.

Vytorin and Zetia are among the top-selling drugs in the world, with combined sales of $5 billion last year. About five million people, including four million Americans, take the medicines, which have been heavily advertised to consumers in the United States.

The New England Journal of Medicine made a similar recommendation about the drugs in an editorial published Sunday.

The panel and the editorial were timed to coincide with the release of full results from a two-year clinical trial that showed that the drugs did not slow, and might speed up, the growth of fatty plaques in arteries. Those plaques are associated with heart attacks and strokes.

Merck and Schering-Plough, which make Vytorin and Zetia, said Sunday that they disagreed with the recommendations. Vytorin and Zetia have been proved to lower cholesterol and are valuable treatments for patients, said Dr. Rick Veltri, vice president of the Schering-Plough Research Institute.

''We feel that nothing's changed,'' Dr. Veltri said.

The stakes of the debate are high both medically and financially. The drugs produce about 70 percent of Schering-Plough's profit, analysts say.

Prescriptions for the medicines have already dipped about 15 percent since January, when preliminary results were disclosed from the trial discussed in detail on Sunday. Still, the drugs are widely used, with about three million prescriptions written each month in the United States alone.

Unlike statins, which block the liver from making cholesterol, Zetia stops the intestine from absorbing cholesterol in food. Vytorin is a single pill that combines Zetia with simvastatin, or Zocor.

Low-density lipoprotein, or LDL, cholesterol, the harmful kind, is a risk factor for heart disease, and so doctors have generally assumed that lowering LDL cholesterol would reduce the risk of heart attacks and strokes.

But proving that a drug actually cuts those risks requires an expensive, multiyear clinical trial involving 10,000 or more patients. Those studies, called outcomes trials, have been conducted for statins, and they have proved that patients taking those drugs have a reduced risk of heart disease. No such outcomes trials exist for Vytorin and Zetia.

The Food and Drug Administration initially approved Zetia in 2002 on the basis of trials that lasted no more than 12 weeks and covered only 3,900 patients.

In 2006, four years after Zetia reached the market, Merck and Schering began enrolling patients in their own outcomes study, which compares people taking Vytorin to those taking Zocor alone.

But the companies said Friday that the results of the trial, which had been expected in 2011, would not come until at least 2012.

As a result, doctors who prescribe the medicines are doing so without hard evidence that they help patients, said Dr. Steven Nissen, chairman of cardiology at the Cleveland Clinic.

''We've got a drug that has no clinical outcome trials,'' Dr. Nissen said. ''I advise my colleagues essentially to use this drug only as a last resort.''

The recommendations released Sunday will probably have an immediate impact on clinical practice, said Dr. Douglas Weaver, the incoming president of the American College of Cardiology, which was host of the conference. Nearly half of the 30,000 cardiologists in the United States are attending the conference, and many of them heard the panel's recommendations firsthand.