Kaiser Permanente, one of the largest managed care organizations, has ordered its pharmacies to stop dispensing Bextra, an arthritis and pain drug made by Pfizer that some tests have indicated may increase the risk of heart attacks and strokes.

Bextra is in the same class of drugs as another Pfizer painkiller, Celebrex, which has also been found to pose cardiac risks at high doses, and as Vioxx, the drug that Merck pulled from the market last fall after its own tests disclosed cardiac risks.

Merck announced yesterday that the Securities and Exchange Commission had opened a formal investigation of Merck's handling of issues related to Vioxx.

The suspension of Bextra is the first case of Kaiser's doctors deciding to ban use of a drug that has been approved by the Food and Drug Administration, said Dr. Sharon Levine, associate executive director of the company's Northern California unit.

Dr. Levine said the decision was unlikely to affect many of Kaiser's nine million patients around the country, since Kaiser physicians had already avoided using the drug. But "for safety and quality reasons, we don't want to be involved in dispensing this drug," she said.

A ban by Kaiser, whose practices are in many ways a model for other health care providers, could further damp sales of Bextra, which had sales last year of $1.29 billion. Those sales have already slumped since Pfizer acknowledged last fall that studies had shown that the drug increased the risks of heart attack and stroke among patients who have had coronary artery bypass surgery. Many experts have speculated that its risks could be as high as Vioxx's.

Kaiser doctors will continue prescribing Celebrex, Dr. David Campen, Kaiser's medical director of pharmacy services, said yesterday. At low doses, Celebrex does not appear to have the same risks as Bextra or Vioxx, Dr. Campen said.

"But clearly at the higher dose levels, there does seem to be a problem" with Celebrex, he said.

A Pfizer spokeswoman said the Bextra tests in question had involved patients after heart surgery, who were not the target population for the drug, which was federally approved to treat arthritis inflammation and pain. "All the available data suggests there is no increased risk of heart attacks among arthritis patients who take Bextra," she said.

Kaiser said that its Bextra ban would take effect on Feb. 1 for new prescriptions and March 1 for refill prescriptions. The lag is intended to give the group enough time to inform the doctors and dentists who see Kaiser patients, Dr. Levine said.

Dr. Levine said that Kaiser took its action against Bextra because the drug had no unique benefits but plenty of possible risks. Clinical trials have found that Bextra cures pain no better than older medicines like ibuprofen and diclofenac. And while Bextra is theoretically supposed to be safer on the stomach than older medicines, this benefit has never been proved in a clinical trial.

Bextra, in rare instances, can also cause a fatal skin rash. Both Bextra and Celebrex should be avoided by people with allergies to sulfa drugs, according to label warnings mandated by the F.D.A.

The F.D.A. has counseled physicians to limit prescriptions of Bextra and Celebrex in the aftermath of studies that found an increased the risk of heart attacks and stroke. The agency has scheduled a three-day hearing of independent experts next month to discuss Vioxx, Bextra and Celebrex. That panel is expected to recommend how the medicines should be regulated.

Merck withdrew Vioxx in September after a study showed that it doubled the risk of heart attacks and stroke. An F.D.A. drug-safety reviewer estimated that as many as 55,000 deaths might have resulted from taking Vioxx, and Merck is facing numerous lawsuits from former Vioxx users or their families.

Beyond disclosing the S.E.C. investigation yesterday, Merck declined to comment beyond a written statement, saying "This action was not unexpected and the company will continue to cooperate with the S.E.C." The S.E.C. also declined to comment.

Analysts say that the inquiry may focus on whether Merck misled its shareholders about the safety of Vioxx. Some internal company documents that have come to light during Vioxx litigation suggest that executives may have known about Vioxx's safety problems several years ago but covered them up.

A Merck spokesman yesterday said that the company "acted responsibly every step of the way, from researching the drug prior to approval to monitoring the drug while it was on the market to voluntarily withdrawing the drug when it did."

The company previously announced that the Justice Department was also investigating the Vioxx case "under criminal statutes."