UPDATE 1-Schering, Merck cholesterol prescriptions slide

(Adds details on anesthesia drug)

NEW YORK, March 18 (Reuters) - U.S. prescriptions of Schering-Plough Corp's SGP.N cholesterol drugs fell 13 percent in February from a month earlier, following results of a controversial study, the drug maker said on Tuesday.

The drugs, Zetia and Vytorin, fell to 2.77 million total prescriptions in February from 3.19 million in January, Schering-Plough said in a filing. Schering-Plough sells the drugs with Merck & Co (MRK.N) in a joint venture.

"Although the prescription data have shown some early signs of stabilization, there are limitations to this prescription data and it is too early to discern any trends," Schering-Plough said.

The company, for which the cholesterol drugs are by far its biggest sellers, said it was too soon to determine the impact of the lower prescription volumes on 2008 results or the longer term.

Shares of Schering-Plough and Merck have fallen about 30 percent each since mid-January, when Vytorin proved no better than generically available Zocor in preventing build-up of fatty plaque in arteries of the neck. Vytorin combines Zetia with Zocor.

The failed trial raised doubts among many patients and doctors about the worth of the costly medicine.

Also on Tuesday, Schering-Plough said U.S. health regulators have extended their review of the company's experimental anesthesia drug, sugammadex, by three months. The extension will allow regulators to review a report related to the potential for allergic reactions.

The Food and Drug Administration is now expected to make a decision on sugammadex in the third quarter. (Reporting by Lewis Krauskopf, editing by Gerald E. McCormick and Dave Zimmerman)