Pfizer Inc. (PFE) said Friday it found an increased risk of heart problems with patients taking its top-selling painkiller Celebrex but added it has no plans follow rival Merck & Co. and pull the drug from the market.

The drug giant said that patients taking Celebrex (search) in a long-term cancer-prevention trial had more than twice the number of fatal or non-fatal heart attacks compared with those taking a placebo. Another trial showed no increased risk.

The heart-attack findings for Celebrex come only 10 weeks after Merck & Co. (MRK) pulled Vioxx (search), a rival drug that is also a COX-2 inhibitor, from the market because it doubled the risk of heart attack and strokes in patients taking the medication longer than 18 months.

Acting Food and Drug Administration (search) Commissioner Lester Crawford said the government is advising physicians to consider prescribing drugs other than Celebrex to their patients.

"We're leaving open all regulatory decisions as we move forward. But we do not have a decision on the fate of the product," said Crawford, during a press briefing. "We do have great concern about this product (Celebrex) and the class of products,"

But doctors have already been inundated with phone calls from worried patients and say they will curtail writing prescriptions for the drug. Shares of Pfizer, a member of the Dow index and the world's largest pharmaceutical maker, plunged $3.23, or 11.15 percent, to $25.75 in late afternoon trading on the New York Stock Exchange (search). The decline wiped out almost $25 billion of Pfizer's market value.

The National Cancer Institute (search), which was conducting the study for Pfizer, suspended the use of Celebrex after discovering that patients taking 400mg to 800mg of the drug daily had a 2.5 times greater risk of experiencing 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. Pfizer was conducting the trials as part of an effort to find a new application for the drug.

COX-2 inhibitors (search) have become popular because of their effectiveness in treating the pain of arthritis and other ailments.

Dr. Garret A. FitzGerald, who has been critical of COX-2 inhibitors, said he doesn't believe Pfizer should take Celebrex off market but has to work hard to find the appropriate patient population for the drug. COX-2s were developed to be gentler on the stomach than older pain relievers called nonsteroidal anti-inflammatory drugs, such as naproxen, that are associated with gastrointestinal problems. But unlike Vioxx, Celebrex was never statistically proven to decrease the risk of ulcers. It also doesn't reduce pain better than older drugs.

"The challenge for Pfizer now is to show why this drug should be chosen," said FitzGerald, a cardiologist at the University of Pennsylvania.

Celebrex is the most-prescribed drug for treating arthritis. 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, accounting for 6 percent of Pfizer's total sales of $37.59 billion.

FitzGerald said Pfizer's huge marketing push behind the drug accounted for its dramatic use. Last year, Pfizer spent $87.6 million to advertise Celebrex. It recently launched a new campaign for the drug and placed full-page ads in newspapers touting Celebrex's safety in the wake of the Vioxx recall.

FitzGerald said he believed the news has implications for COX-2 inhibitors such as those under development at Merck and Novartis.

"I think the trial concludes the controversy about whether there is a class effect of these drugs. Now there is clear evidence of it," said FitzGerald. "You would need to believe the earth is flat if you thought this was just a coincidence."

Barbara Ryan, a managing director at Deutsche Bank said she expects Celebrex's sales to fall by 50 percent next year and has dropped her Pfizer 2005 earnings estimate to $2.10 a share from $2.35 a share.

"In this environment people are hysterical," said Ryan, who said all the headlines would scare people even though the drug's problem manifested itself at high doses.

Moody's Investors Service (search) revised its outlook on Pfizer to negative from stable because it believes Celebrex use may decline as the controversy about the class increases. Moreover, it said the likelihood that the drug could be pulled from the market may have risen and Pfizer's litigation may increase.

Earlier this month, the FDA said it was adding a warning to the labels of another Pfizer drug, Bextra, noting a risk of potential heart problems associated with its use in people who have recently had heart bypass surgery. Bextra is also a COX-2 inhibitor.

Also on Friday, Eli Lilly & Co. (LLY) said it is warning doctors to stop using its attention deficit disorder drug Strattera in patients with jaundice or laboratory evidence of liver injury. Lilly said it was putting a boldface warning in the prescribing information that it gives doctors about the drug after two patients on the medication developed liver problems.

Reuters and the Associated Press contributed to this report.