The Federal Trade Commission said Monday it filed a complaint against Schering-Plough ( SGP), Upsher-Smith and American Home Products ( AHP), accusing the drug makers of entering into an illegal anticompetitive agreement to keep low-cost, generic drugs off the market.

In an administrative complaint, the FTC claimed Schering-Plough illegally paid Upsher-Smith and American Home Products millions to delay the launch of their generic versions of K-Dur, a widely prescribed potassium chloride supplement made by Schering-Plough. The commission alleges the agreements have cost consumers more than $100 million.

A federal administrative law judge will hear the FTC's complaint.

Schering-Plough lost 53 cents, or 1.4%, to $36 in recent trading on the New York Stock Exchange. American Home Products fell $1.26, or 2.1%, to $57.49.