TRENTON, N.J. – The makers of the blockbuster arthritis drug Vioxx, taken by millions for pain and seen as a potential cancer prevention medicine, pulled it from the market Thursday after a study found it doubled the risk of heart attacks and strokes.

Medical experts advised patients to stop taking Vioxx and consult their doctors about alternatives. Merck & Co., the drug’s maker, said about 2 million people worldwide use Vioxx, and a total of 84 million have taken it since it came on the market with great fanfare in 1999.

News of the drug’s dangers came from a three-year study aimed at showing that Vioxx could prevent the recurrence of polyps, which can turn cancerous, in the colon and rectum. Merck stopped the study after discovering study participants had double the risk of a heart attack, compared to others taking dummy pills.

The company said the heart risks and other cardiovascular complications appeared 18 months after patients started taking Vioxx, which also is prescribed for acute pain and disorders such as carpal tunnel syndrome.

“We’re taking this action because we believe it best serves the interest of patients,” said Merck’s chairman, president, and chief executive, Ray V. Gilmartin, in a prepared statement.

The Food and Drug Administration said there were early signs of potential problems with Vioxx. A Merck study led to warnings about heart risks being placed on the drug’s label in 2001, and the FDA has been monitoring problems reported to it since then.

“This is not a total surprise,” said the acting director of the FDA’s Center for Drug Evaluation and Research, Dr. Steven Galson.

Despite that, the FDA on September 8 approved use of Vioxx for juvenile rheumatoid arthritis in children as small as 2 years old and 22 pounds. FDA did not immediately respond to a request for comment on that decision.

Officials don’t know yet how Vioxx may be causing the increased heart risk.

It could be because Vioxx seems to increase chances of developing high blood pressure more than other anti-inflammatory drugs, said the director of rheumatology at New York University Hospital for Joint Diseases, Dr. Steven Abramson.

“There are very few patients for whom there won’t be a good alternative drug,” Dr. Abramson said.

Besides generic anti-inflammatory drugs such as ibuprofen, naproxen, and aspirin, those include Pfizer’s blockbuster Celebrex, which Dr. Abramson said has not been linked to heart complications.

“A person doesn’t have to worry because they took a pill today or yesterday that they’re going to have a heart attack,” he added.

Vioxx’s removal also will be a blow to hopes that it and other drugs known as cox-2 inhibitors could be used to prevent cancer in people at high risk of developing it. A landmark study in 2002 showed that small, daily doses of aspirin could prevent colon cancer, and studies hinted that cox-2 inhibitors might do the same, possibly without aspirin’s side effects.

All cox-2 inhibitors can raise blood pressure, but Vioxx appears to be the only one that’s been linked to higher risk of heart attacks and strokes, said Dr. Galson.

Dr. Abramson said that while the cox-2 inhibitors protect the stomach more than the older anti-inflammatory drugs, they all have a comparable effect on pain. But he said that for specific patients, different drugs relieve pain more or cause less side effects such as gastrointestinal ulcers, stomach cramping and diarrhea.

By yesterday afternoon, plaintiffs’ attorneys were already circling Merck, with at least one announcing plans for a class-action lawsuit. Another claimed he already represents 58 patients around the country allegedly harmed by Vioxx, including people who suffered a heart attack, stroke, internal bleeding, or kidney failure.

“It’s a disaster for Merck, coming at the worst time,” said independent health care analyst Hemant Shah of HKS & Co. in Warren, N.J.

Vioxx is one of Merck’s most important drugs, with $2.5 billion in sales in 2003. But sales dipped 18% in the second quarter of this year to $653 million, partly due to increasing concerns about the drug’s safety.

Vioxx is costly, with a 30-day supply of 25-milligram pills costing about $100, far more than the generic anti-inflammatory medicines. Merck said patients with remaining pills can mail them back to the company for a refund.

Merck, the world’s third-biggest drug maker, announced the news before the stock market opened. Merck shares closed down $12.07, or 26.78 percent, to $33 with more than 100 million shares traded.

The analyst Mr. Shah said the withdrawal of Vioxx comes “at a time when they really need to get ready for expiration” of its patent for Zocor, a high cholesterol drug which is Merck’s top-seller.

Zocor loses patent protection early in 2006 and sales are expected to plunge when generic competition begins. In an effort to replace those revenues, Merck recently launched a drug with partner Schering-Plough Corp., Vytorin, that combines Zocor and Schering-Plough’s Zetia to attack cholesterol levels in two complementary ways.

“This makes it almost inevitable for the company to find a merger partner for them to continue to grow,” Mr. Shah said.

Merck’s announcement stands to benefit rival Pfizer Inc., the world’s biggest drug maker. The two companies have been battling for market share, with Pfizer’s Celebrex dominating the market with about $5 billion in American sales alone last year.

“I think Celebrex sales are going to significantly increase,” Mr. Shah said.

Vioxx and a successor drug called Arcoxia, approved in some other countries and awaiting Food and Drug Administration approval here, are part of a class of anti-inflammatory drugs heavily touted by the pharmaceutical industry as being more effective and having less side effects, particularly on the gastrointestinal system, than older drugs.

Pfizer’s Celebrex and its successor drug, Bextra, which already is on the market in America, also are cox-2 inhibitors.

Merck said the Vioxx recall will slash about 50 cents to 60 cents a share from its earnings for the rest of this year. That includes foregone sales, write-offs of inventory held by Merck, customer returns of product previously sold and other costs of the pullback. Merck expects foregone fourth quarter sales of Vioxx of $700 million to $750 million alone.

Merck, which is based in Whitehouse Station, N.J., had previously been expecting 2004 earnings per share of $3.11 to $3.17.

Merck is scheduled to release financial results for the third quarter, which ended yesterday, on October 21.