Merck's Vioxx Gets OK for Child Arthritis

Meanwhile, Merck will be sweating out another insurance company's comparison test.

An impending review by Aetna(AET Quote) worries David R. Risinger of Merrill Lynch, who recently issued a report saying he expected the Aetna results also to show an elevated risk of "acute cardiac events" of Vioxx vs. Celebrex and other drugs that treat arthritis and relieve pain.

"We believe another negative study for Vioxx in such a short period following the Kaiser study could accelerate market share loses for Vioxx beyond what we recently forecasted," he said in an Aug. 30 report in which he reiterated his neutral rating on Merck.

"While retrospective studies do not provide definitive evidence of a cause-and-effect relationship between Vioxx and cardiac events, the 'headline' effect from the reporting of these studies could increase concerns among payers, patients and physicians, negatively affecting the brand," said Risinger. (His firm is a market maker in the stock and has a non-investment banking relationship with Merck. His research report says one or more members of his analyst team own shares in Merck.)

Merck's stock has been essentially flat since the FDA-sponsored study report was issued at a medical conference in France just over two weeks ago. On Wednesday, the stock was trading at $45.68, down 10 cents.