WASHINGTON — The Food and Drug Administration on Friday warned physicians to consider alternatives to the popular arthritis drug Celebrex because of new evidence that, like the similar drug Vioxx, removed from the market in September, it doubles the chances of heart attacks and strokes.

The federal agency said it may soon take other actions, including requiring stronger warnings, or even blocking the drug from sale. The statements call into question the future of the widely used class of painkilling drug called Cox-2 inhibitors.

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The FDA warning came just hours after Celebrex manufacturer Pfizer Inc. issued a statement saying that a study of the drug's efficacy as a cancer treatment found evidence of increased cardiovascular risk.

The study's findings added to the growing concerns about the effectiveness of the FDA's system for assessing the risks of new drugs and for monitoring dangerous side effects in medications it had already approved.

"Right now we have a situation where the public is left wondering when the next shoe might drop when it comes to drug safety," said Sen. Charles E. Grassley (R-Iowa), who held hearings last month on the problems with Vioxx. "At this point, no one can say with confidence whether the worst drug safety problems are behind us or ahead of us."

But Dr. John Jenkins, director of the FDA's Office of New Drugs said discovery of the problems with Celebrex is a sign that the system is working.

"I don't see this as a failure of the system," Jenkins said. "This is how drug development, drug approval and post-market monitoring is done, not only in the United States, but in all countries that have a regulatory system."

Pfizer gave no indication that it would withdraw the drug.

Dr. Joseph Feczko, president of worldwide development for Pfizer, said in a statement that doctors "should factor this new information" and weigh it against the risks of other painkilling drugs.

But patient groups voiced concern about the continued use of Celebrex and related drugs.

The Arthritis Foundation issued a statement Friday urging caution in the use of any Cox-2 inhibitor, a class of drug that targets an enzyme that causes inflammation but ignores another that protects the stomach lining.

That position was bolstered Friday with the release of a letter from three physicians at the Vanderbilt University School of Medicine in Tennessee warning doctors against prescribing another widely used Cox-2 inhibitor, Bextra, "except in extraordinary circumstances."

The letter, which will be published in the Thursday edition of the New England Journal of Medicine, was released early because of Pfizer's Celebrex announcement.

The doctors said that previous findings of an increased risk for heart bypass patients taking Bextra, which is also made by Pfizer, were serious enough to call into question the drug's safety for other people.

"This is a doomed class of drugs," said Dr. Sidney Wolfe, director of health research for the consumer advocacy group Public Citizen. "They will be taken off the market. It's a matter of when, not whether."

Celebrex and other Cox-2 inhibitors were developed to be a stomach-friendly alternative to old line anti-inflammatory medicines such as aspirin, ibuprofen and naproxen. Taken long term, the older medicines can cause gastric problems such as ulcerations and internal bleeding.

The new Cox-2 inhibitors became tremendous money makers because they are used as maintenance drugs, taken on a daily basis by millions of people worldwide. They sell for as much as $3 a pill. The older medicines sell for pennies a pill in generic versions sold in supermarkets and drug stores.

Celebrex and Bextra were expected to account for $5 billion of Pfizer's estimated 2005 sales of $55 billion, said Sena Lund a market analyst for Cathay Financial in New York. Roughly 27 million patients have taken Celebrex, according to Pfizer.

But in September, a serious problem emerged with one of the Cox-2 inhibitors. Merck & Co. pulled Vioxx off the market after it was found to cause heart attacks in some patients.