Vioxx, a heavily advertised and popular arthritis pain medication, was withdrawn from the market this morning by pharmaceutical giant Merck and Co., based on new study suggesting that Vioxx may increase the risk of heart attack and stroke.

Merck, along with the U.S. Food and Drug Administration, urged patients using Vioxx to contact their health care providers to discuss discontinuing use and possible alternative treatments.

The FDA said it would closely examine other drugs in the same class as Vioxx to determine whether they also can cause heart attacks and strokes.

In a statement issued earlier, the FDA said that patients taking Vioxx regularly "face twice the risk of a heart attack compared to patients receiving a placebo."

Vioxx is in a class of painkillers known as COX-2 inhibitors, which also includes Pfizer's Celebrex and Bextra, an experimental drug called Prexige made by Novartis AG and another Merck drug called Arcoxia that the company has been seeking FDA approval to market.

In a press release, Merck said its data did not necessarily implicate any other company's drug.

Steven Galson, acting director of the FDA's Center for Drug Evaluation and Research, told reporters in a conference call that the agency is "going to be more interested in looking at long-term data on new products that come down the pike." He said he was referring to COX-2 inhibitors now on the market as well as those awaiting FDA approval.

The news drove Merck's stock sharply downward, slashing its market value by $25 billion, the Reuters news agency reported. Vioxx brings in more than $2.5 billion annually for Merck and constitutes roughly 10 percent of the pharmaceutical company's worldwide sales.

Some previous studies have found a link between Vioxx and heart risk. In June, 2000, Merck had alerted the FDA to the problem. But the FDA said today that after reviewing the results at that time, it determined that a warning on the package would suffice.

Vioxx has been on the market since 1999 and is sold in 80 countries. In some countries the drug is called Ceoxx rather than Vioxx. In the United States, at least 23.5 million prescriptions for Vioxx were written in 2003, according to IMS Health, a pharmaceutical information and consulting company.

Merck said its actions today came after it found increased risk for cardiovascular problems after prolonged use in a trial testing Vioxx as a preventive for colorectal polyps.

"In this study," it said today, "there was an increased relative risk for confirmed cardiovascular events, such as heart attack and stroke, beginning after 18 months of treatment in the patients taking Vioxx compared to those taking placebo."

The company said that "the results for the first 18 months of treatment did not show any increased risk."

The trial was halted after the finding.

"We are taking this action because we believe it best serves the interest of patients," said a written statement from Raymond Gilmartin, chairman and chief executive of Merck. "Although we believe it would have been possible to continue to market Vioxx with labeling that would incorporate these new data, given the availability of alternative therapies, and the questions raised by the data, we concluded that a voluntary withdrawal is the responsible course to take."

The FDA issued a public health advisory suggesting that patients using Vioxx consult their physicians.

"Merck did the right thing by promptly reporting these findings to FDA and voluntarily withdrawing the product from the market," said a written statement by FDA Commissioner Lester M. Crawford. "Although the risk that an individual patient would have a heart attack or stroke related to Vioxx is very small, the study that was halted suggests that, overall, patients taking the drug chronically face twice the risk of a heart attack compared to patients receiving a placebo."

The company said in a statement that its fourth quarter sales would suffer to the tune of $700 million to $750 million, knocking down earnings per share by 50 or 60 cents. Merck's shares plummeted in trading after the announcement, as did the Dow Jones industrial average, which includes Merck as a component.

The COX-2 drugs are designed to fight inflammation and pain while reducing the risk of ulcers caused by over-the-counter pain medication, such as aspirin and ibuprofen. But some studies have found that COX-2 inhibitors might cause blood clots that trigger heart attack and stroke. Some subsequent studies showed Vioxx carries greater heart risk than other drugs in its class.