Merck Down on Arthritis Study

updated from 11:39 a.m. EDT

Merck (MRK) shares fell Thursday after a study sponsored by the Food and Drug Administration concluded that its Vioxx arthritis drug put users at a greater risk of heart attacks than Pfizer's (PFE) Celebrex.

As a result, one of the nation's biggest health-maintenance organizations is reconsidering its use of the drug.

Merck shares lost 96 cents, or 2.1%, to $45.06 in midmorning trading. Pfizer shares rose 15 cents, or 0.5%, to $32.06.

Details of the study, which showed Vioxx users had a 50% greater chance of heart attacks and sudden cardiac death than those using Celebrex, were first reported Wednesday. Kaiser Permanente, which contributed resources to the study that was presented at a medical conference in France, confirmed its decision to review the drug.

Merck Thursday released a detailed critique, questioning the study's significance.

"This analysis is a retrospective database analysis -- not a clinical trial," Merck said. "Observational analyses have limitations, often conflict with each other and must be interpreted within the context of data from large, randomized, controlled clinical trials."

The company said that such randomized, controlled clinical trials are the "gold standard" in evaluating a drug's safety and efficacy. The company added that although the study was financed by the FDA under contract with Kaiser, "the conclusions presented by the authors do not necessarily reflect the views of the FDA."

The study clearly represents a negative impact for Merck, said John T. Boris, a drug industry analyst for Harris Nesbitt, in a report to clients Thursday. "We believe the results from this study provide an additional quiver in the arrow of several thousand Pfizer sales representatives as they continue to chip away at Merck's Vioxx franchise," he wrote.