By Kim Dixon

WASHINGTON (Reuters) - A Schering-Plow Corp (SGP.N: Quote, Profile, Research) drug to reverse the effects of anesthesia appears to be generally safe, U.S. regulatory staff said, but they questioned how the company defined effectiveness in a key study.

Regulators are also awaiting data on a rare occurrence of allergic reactions in some patients using the drug, to be called Bridion, Food and Drug Administration staffers wrote in documents posted on the FDA's Web site on Friday.

The FDA staff said: "The Applicant appears to have made a thorough assessment of the safety of Bridion in a relatively healthy adult population and with the possible exception of hypersensitivity, found the drug product to be safe" compared with current agents.

Schering-Plow shares were down 31 cents or 1.6 percent to $19.72 in afternoon trading on the New York Stock Exchange.

Chief executive Fred Hassan has said the drug could be a growth driver going forward and some analysts have said that, if approved, it could generate up to $1 billion a year in revenue.

"Expectations for this drug are quite high so any concern raised is appropriately making people nervous," said Barbara Ryan, an analyst at Deutsche Bank. But she expects the panel to see that the benefits outweigh the risks.

The review comes ahead of an advisory panel meeting next week to consider the risks and benefits of the drug, known generically as sugammadex. The FDA typically takes the advice of its panels.

The drug seeks to reverse the muscular blocking effects of muscle relaxants, to help patients recover more quickly from anesthesia after medical procedures.