Updated from 1:10 p.m. EST

Pfizer ( PFE) shares plunged in frenetic trading volume Friday after the drugmaker said new data from cancer trials showed its arthritis drug Celebrex posed "increased cardiovascular risk" when used in high doses.

Celebrex is a COX-2 inhibitor drug like Merck's ( MRK) Vioxx, which that drugmaker withdrew from the market in late September because of similar health and safety concerns.

Pfizer said it received new information last night about two long-term cancer trials concerning the cardiovascular safety of Celebrex. The other one "revealed no greater cardiovascular risk than placebo."

Celebrex is approved for use in the U.S. for the treatment of arthritis and pain. Pfizer has stood by the safety of its use since the Vioxx recall, which has knocked some 40% off Merck shares, and repeated that it had no plans to pull the money-spinning drug from the market. The Food and Drug Administration, however, responded to the development by recommending that physicians advise patients to use the lowest dose possible.

Shares of Pfizer hit a new 52-week low of $25.76, down $3.22, or 11.1%. Shares fell as low as $23.52. About 277 million shares had already changed hands -- about nine times the average daily volume of about 31 million shares.

Pfizer stressed the importance of "placing this new information in context," because the trial results differ from "the large body of data that we and others have accumulated over time, in which an increased risk of serious cardiovascular events in arthritis patients, even at higher-than-recommended doses, had not been seen."

"These clinical trial results are new. The cardiovascular findings in one of the studies (APC) are unexpected and not consistent with the reported findings in the second study," the company said in a statement. "Pfizer is taking immediate steps to fully understand the results and rapidly communicate new information to regulators, physicians and patients around the world."

The Vioxx recall has shaken the once mighty Merck. The company earlier this week said that as of Oct. 31, some 475 personal injury lawsuits have been filed against the company relating to its withdrawal of Vioxx from the market on Sept. 30. Product liability cases include 1,100 plaintiff groups.

At this point, the two cases are dramatically different, but the cancer trial data are bound to renew speculation about the safety of Celebrex for treating arthritis.

In mid-October, Pfizer announced plans to fund additional clinical testing of Celebrex to determine its effect on arthritis patients who have had a recent heart attack.

Another Pfizer COX-2 inhibitor drug, Bextra, has recently drawn attention because of possible side effects, including a rare but potentially fatal skin rash, as well as some research that says Bextra patients undergoing cardiac bypass surgery had a higher risk of heart-related complications. Pfizer has released research showing that there are no increases in cardiovascular risks among patients who took Bextra for between six weeks and 52 weeks.

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