Pfizer Inc. says it has found an increased risk of heart problems with patients taking its painkiller Celebrex, a drug that is in the same class as Vioxx, which was pulled from the market in September because of safety concerns.

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NEW YORK — Pfizer Inc. says it has found an increased risk of heart problems with patients taking its painkiller Celebrex, a drug that is in the same class as Vioxx, which was pulled from the market in September because of safety concerns.


Pfizer announced today that it found the increased risk in one of two long-term cancer-prevention trials, while the other trial showed no increased risk. The company was conducting the trials as part of an effort to find a new application for the drug.


Pfizer did not indicate that it was withdrawing Celebrex from the marketplace.


The news sent shares of the giant pharmaceutical maker plunging in early trading on the New York Stock Exchange, where its shares were down $4.37, or 15 percent, at $24.61.


Celebrex is currently approved for use in the United States for the treatment of arthritis and pain.


The National Cancer Institute, which was conducting the study for Pfizer, suspended use of the drug after finding that patients taking 400mg to 800mg of Celebrex daily had a 2.5 times greater risk of major heart problems than those who were not. A separate cancer study found no increased heart risk with patients taking 400mg of Celebrex per day.


Earlier this month, the Food and Drug Administration said it was adding a warning to the labels of another Pfizer drug, Bextra, warning of potential heart problems associated with the use of that drug in people who recently had heart-bypass surgery.


Bextra, Celebrex and Vioxx are all in a class of pain drugs called cox-2 inhibitors. The drugs have become popular because of their effectiveness in treating the pain of arthritis and other ailments.


Merck & Co. pulled Vioxx from the market Sept. 30 after a study indicated it doubled the risk of heart attacks and stroke when taken for longer than 18 months.


Studies done five years ago — when Celebrex and Vioxx were approved — suggest the same mechanism that inhibits inflammation and makes the drugs easier on the stomach also blocks a substance that prevents heart problems, scientists have said.


The withdrawal of Vioxx has been a financial and public-relations disaster for Merck. Its legal liabilities are estimated at up to $18 billion, and its shares have dropped by nearly one-third since the recall announcement was made in late September.


Vioxx had been a blockbuster drug for Merck, it’s No. 2 earner with annual global sales of $2.5 billion. Some 2 million people worldwide had been taking Vioxx.


In the nine months ending in September, worldwide sales of Celebrex more than doubled from the same period a year earlier to $2.29 billion.


Dr. Joseph Feczko, president of worldwide development for Pfizer, noted that the results in the trial finding increased risk of heart attacks were not consistent with either the other cancer-prevention trial or with a “large body of data” that the company had collected.


“Pfizer is taking immediate steps to fully understand the results and rapidly communicate new information to regulators, physicians and patients around the world,” Pfizer’s chief executive Hank McKinnell said in a statement.