Pfizer ( PFE) didn't have much time to take advantage of Merck's ( MRK) misery after withdrawing its painkiller Vioxx from the market. Two of Pfizer's arthritis drugs, which belong to the same class as Vioxx, are now under regulatory and investor scrutiny. Here's a list of key events and dates.

Sept. 30: Merck pulls Vioxx from the market because a test showed that the drug caused a significant increase in cardiovascular risk among people who took the drug for more than 18 months.

Oct. 1: Pfizer says three large long-term Celebrex studies involving more than 6,000 patients "have not shown any significant safety issues and are expected to continue to completion." Pfizer adds that "the data we've accumulated over time demonstrate that Celebrex does not increase the risk of serious cardiovascular events in patients with arthritis and pain, even at higher-than-recommended doses." Celebrex reached the U.S. market in 1998, a year before Vioxx was available.

Oct. 7: An editorial published in an online version of The New England Journal of Medicine raises questions about COX-2 inhibitors in general. Dr. Garret A. FitzGerald, a medical researcher at the University of Pennsylvania, says the FDA should examine all COX-2 drugs. "We must remember that the absence of evidence is not the evidence of absence," FitzGerald says. He doesn't advocate banning all COX-2 drugs for all patients. These drugs "remain a rational choice for patients at a low cardiovascular risk who have had serious gastrointestinal events, especially while taking traditional" pain relievers. He says it would "seem prudent" to avoid prescribing COX-2 drugs for patients "who have cardiovascular disease or who are at risk for it."

Oct. 15: Pfizer says it is working with regulatory agencies "around the world" to update the label of its COX-2 drug Bextra. Pfizer would later reveal that this update is a black box warning -- the toughest warning required by the FDA -- for a rare but potentially fatal skin rash called Stevens-Johnson syndrome. A warning about this side effect had been on the Bextra label since 2002, a year after Bextra reached the U.S. market. This side effect exists with other medications, including other COX-2 drugs, but it occurs more often with Bextra.

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