Some analysts are betting Pfizer Inc. (PFE) won't have to withdraw its arthritis drug Celebrex but they do expect sales to fall after a study of the drug in cancer patients showed it increased the risk of heart attacks.

Pfizer has said it has no plans to withdraw Celebrex (search), which had annual sales of nearly $2 billion last year. The Food and Drug Administration (search), which on Friday urged doctors to consider prescribing alternatives to Celebrex, plans to make additional announcements about the drug over the next few days.

Several analysts believe Celebrex is mostly safe when used at the typical levels of 100 milligrams or 200 milligrams. The trial of patients with tumors known as adenomas took either 400 milligrams or 800 milligrams of the drug. Previous trials of Celebrex in arthritis and Alzheimer's have not shown an increase in heart risk.

Two-thirds of patients in the cancer trial took the drug for two years, some up to four years. Merck & Co. Inc.'s (MRK) recently withdrawn arthritis drug Vioxx (search), which is similar to Celebrex, showed it doubled the risk of heart attack and stroke, but the risk only emerged after patients had been taking the drug for 18 months or more.

"We do not believe there will be a withdrawal at this point," said Winton Gibbons, an analyst at William Blair & Co. "Our read of the data so far suggests that high dosing is the primary risk driver, with duration of therapy possibly exacerbating the risk."

Pfizer's shares fell 11.2 percent on Friday after it said a trial sponsored by the National Cancer Institute showed patients with adenomas that grow from glandular tissue had more than twice the risk of heart attacks than those taking a placebo. On Monday they fell an additional 5.2 percent to $24.41 on the New York Stock Exchange (search).

"We believe the product will be able to stay on the market because the studies only show a safety problem in high risk cancer patients," said David Moskowitz, an analyst at Friedman, Billings, Ramsey.

The FDA has asked Pfizer to temporarily stop advertising Celebrex. The drug has been one of the most heavily advertised in pharmaceutical history.

Concerns are not limited to Celebrex, however. Question marks also hang over Pfizer's Bextra (search), another COX-2 inhibitor (search).

But George Grofik, an analyst at Smith Barney Citigroup, said he puts the risk of a worldwide withdrawal of Pfizer's COX-2 franchise at about 30 percent. "COX-2s may have legitimate benefit in a small sub-population of patients at higher gastrointestinal risk," he said in a report.

However, he said "it is clear COX-2 use is likely to decline precipitously after this news," and cut his earnings forecast to $2.05 from $2.29.

Gastrointestinal bleeding and ulcers can be caused by older painkillers used to treat arthritis. COX-2s were designed to prevent that.

Still, some analysts say the damage to Celebrex and Bextra is likely to be so severe that even if they stay on the market sales will be minimal.

"We have removed Celebrex and Bextra from our models, reflecting irreparable harm commercially, and possibly market withdrawal," said Steve Scala, an analyst at SG Cowen.

No matter what the studies show, though, some patients will want to keep taking the drug, just as some wanted to stay with Vioxx, said John Klippel. president of the Arthritis Foundation (search).

"I think there is likely to be many people who even given that knowledge would be likely to want to continue to take Celebrex," he said.