Some hope for Merck and Schering's sales may come from Impact Rx, a prescription data service that bases its predictions on the behavior of a select group of doctors who prescribe high volumes of drugs.

FORBES: Waiting For ENHANCE

That's the picture from newly released data from IMS Health (nyse: RX - news - people ), a pharmaceutical information and consulting company that independently keeps track of how often medicines are prescribed and how much revenue they are generating.

FORBES: The Best-Selling Drugs In America