Merck & Co.'s Zetia failed to outperform Niaspan, a modified form of vitamin B, in reducing plaque buildup in arteries and boosting the level of HDL, or "good" cholesterol, in a study of 200 patients taking statins. Merck officials and critics of the study downplayed the findings, saying the trial examined only the amount of plaque on artery walls, a surrogate marker, and not the rate of heart attack or stroke.

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