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By Toni Clarke

BOSTON, Nov 16 (Reuters) - Shares of Merck & Co Inc (MRK.N) rose on Monday after results of a clinical trial of its cholesterol drug Zetia proved to be not as negative as some analysts had feared.

Results of the trial, known as ARBITER 6, were released on Sunday, and showed Abbott Laboratories Inc's (ABT.N) drug Niaspan appeared more effective than Merck's Zetia as a supplementary cholesterol treatment to statins such as Pfizer Inc's (PFE.N) Lipitor.

But editorials in the New England Journal of Medicine questioned the study and its conclusions, saying it was terminated prematurely and the small number of patients studied precluded a definitive conclusion that niacin, the active ingredient in Niaspan, should be the preferred supplementary agent.

"We, and many others, have felt that the ARBITER 6 deck was stacked in favor of Niaspan, meaning it was no real surprise that it won," said Tim Anderson, an analyst at Sanford Bernstein. "The two editorials in the NEJM -- neither of which was written by the Cleveland Clinic's Dr. Steve Nissen, which had been feared -- are quick to bring balance to the discussion of ARBITER 6's findings."

Nissen, one of the most influential cardiologists in the country and a long-time critic of Zetia, is of the opinion that the key test of the drug's efficacy should be whether or not it reduces heart attacks and strokes.

Zetia has been shown to reduce cholesterol, but it is an open question whether that translates into reduced cardiovascular events. Merck is in the midst of conducting such a study.

Some analysts do not think sales of Zetia will be dramatically affected.

"We expect limited commercial impact to the cholesterol franchise and believe the ARBITER overhang on Merck shares will lift following today's balanced editorials," said Chris Schott, an analyst at J.P. Morgan, in a research note.

But just hours after the release of the data, some physicians are being flooded with calls from concerned patients.

"I can tell you we're already getting swamped by patients who want to stop taking Zetia," said Dr. Micah Eimer, associate professor in the department of medicine at the University of Chicago. "It's getting harder and harder to stand our ground and say, 'Keep taking Zetia,' and to be honest, I'm not prepared to keep doing it."

The study tested the effect on carotid artery wall thickness of adding either Niaspan, which raises good HDL cholesterol, or Zetia, which lowers bad LDL levels.

Increases in thickness of the artery wall could be an indicator of a build-up of plaque, which can cause heart disease.

Merck's shares rose 2.3 percent in afternoon trading to $33.87 on the New York Stock Exchange. Shares of Abbott rose 1.6 percent to $53.82. (Reporting by Toni Clarke and Bill Berkrot, editing by Dave Zimmerman and Gerald E. McCormick)

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