New prescriptions for Pfizer Inc.'s (PFE) arthritis drug Celebrex fell 56 percent last week after the company released data showing the drug may increase the risk of heart attacks and strokes, according to Verispan, a market research company.

New prescriptions for naproxen fell 33 percent in the week ending Dec. 24 after the National Institutes of Health (search) linked it too to possible heart risks.

"The data reflects the concern that exists among doctors and patients and is not at all unexpected," said Mariann Caprino, a spokeswoman for Pfizer.

The company's shares fell 0.7 percent.

A drug called Mobic, sold by Boehringer Ingelheim and Abbott Laboratories Inc. (ABT) are among those picking up patients who are switching from Celebrex. Its new prescriptions gained 6.8 percent during the same period. Abbott's stock rose 0.9 percent.

Celebrex's share of the new prescriptions in the arthritis painkiller market fell to 7.9 percent last week from 14.9 percent the week earlier. Mobic increased its market share to 9.2 percent from 7.1 percent.

Celebrex is a member of a class of drugs called COX-2 inhibitors (search), which also includes Pfizer's drug Bextra and included the withdrawn drug Vioxx. Bextra has also recently been linked with heart problems in high-risk patients.

Bextra's share of new prescriptions during the week fell to 4.7 percent from 5.7 percent the week before.

COX-II's are a subset of a group of drugs called non-steroidal anti-inflammatory drugs (search) (NSAIDs) that relieve pain. Mobic, ibuprofen, naproxen and aspirin are also NSAIDs.

Pfizer's shares were off 19 cents at $27.07 on the New York Stock Exchange. Abbott's shares were up 41 cents at $47.25.