Pfizer Inc. (PFE)’s market share for the cholesterol pill Lipitor (PHAM1150), now at 41 percent after losing patent protection last year, will decline further in the next five months, according to generic-drug maker Watson Pharmaceuticals Inc. (WPI)

Watson, producer of a Pfizer-authorized copy of Lipitor, competes with a generic version made by India’s Ranbaxy Pharmaceuticals Ltd. Watson and Ranbaxy entered the U.S. market on Nov. 11, and have exclusive rights for 180 days before other manufacturers can sell copies.

Paul Bisaro, Watson’s chief executive officer, today revised a forecast he made last year that New York-based Pfizer would hold onto 40 percent of the market once the copycats went on sale. Pfizer gets revenue from Parsippany, New Jersey-based Watson’s version.

“Their ability to maintain that 40 percent will probably erode,” Bisaro said at an investor conference held by Goldman Sachs Group Inc. in New York. “History would indicate that it would come down a little bit.”

Pfizer’s share of the market was 41 percent as of Dec. 24 for the second week in a row, according to an report from Bloomberg Industries.

Pfizer fell less than 1 percent to $21.60 at the close of New York trading. Watson gained 2.8 percent to $61.99.

To contact the reporter on this story: Drew Armstrong in Washington at darmstrong17@bloomberg.net;

To contact the editor responsible for this story: Reg Gale at rgale5@bloomberg.net