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

The federal agency also said it might 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 pain-killing drug called COX-2 inhibitors.

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 already has 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 was a sign that the system was working.

“I don’t see this as a failure of the system,” Jenkins said.

Pfizer gave no indication that it would withdraw the drug.