* Study compared Zetia with Abbott's Niaspan

* Stopped early for unspecified reasons

* Merck falls 3.8 pct; Schering, Abbott also down (Adds Merck does not know study results, why study stopped)

By Lewis Krauskopf

NEW YORK, July 9 (Reuters) - Merck & Co Inc (MRK.N: Quote, Profile, Research) shares fell 3.8 percent on Thursday amid speculation its Zetia cholesterol medicine fared poorly in a clinical trial comparing it with Abbott Laboratories' (ABT.N: Quote, Profile, Research) Niaspan in preventing plaque build-up in arteries.

The study was stopped early for unspecified reasons, according to a posting on a federal clinical trials website (here) although the posting did say it was not stopped because of safety concerns.

The study evaluated the impact of the drugs on atherosclerosis by measuring change in thickness of the carotid artery. Zetia cuts levels of bad "LDL" cholesterol, while Niaspan -- an extended-release version of niacin -- raises good "HDL" cholesterol.

Natixis Bleichroeder analyst Jon LeCroy downgraded his rating on Merck from "buy" to "hold," saying he suspected the early termination means Niaspan may have worked better than Zetia.

"We are now assuming that this trial significantly favored Niaspan and, as a result, we are decreasing our sales estimates for both Zetia and Vytorin," LeCroy said in a research note.