A federal judge blocked sales of a generic version of blood thinner Plavix Thursday while he considers the merits of a claim that a Canadian pharmaceutical company improperly entered the market.

U.S. District Judge Sidney H. Stein issued his written ruling Thursday after a two-day hearing in which Sanofi-Aventis and its partner Bristol-Myers Squibb Co. (BMY) insisted that the Canadian company, Apotex Inc., must be forced to stop selling its product. Stein denied demands by Bristol-Myers and Sanofi-Aventis (SNY) that Apotex be forced to withdraw from the market the generic product it had already sold.

Apotex had argued that a patent protecting sales of Plavix from competitors was no longer valid and that it was in the public interest to permit sales of its generic version of the drug because lower prices mean even more people can use a lifesaving drug.

The Ontario-based Apotex began selling its product Aug. 8 after negotiations between the three companies failed to settle the dispute.

Stein said he based his decision on the chances that Apotex could ultimately prove it is entitled to sell its generic drug; the level of irreparable harm each side might suffer; the balance of hardships; and the impact on the public.

Bristol-Myers disclosed last month that the Department of Justice was investigating a deal in which Bristol-Myers and Sanofi-Aventis agreed to pay Apotex at least $40 million to keep its version of Plavix off the market until 2011. The deal was rejected by state attorneys general.

At the hearing before Stein, attorney Evan Chesler of Sanofi-Aventis that allowing Apotex to flood the market with its product and force prices lower would cripple the spirit of drug innovators, jeopardizing future pioneering medical breakthroughs.

"You can never put this Humpty-Dumpty back together if it's not stopped," Chesler said.

Frank Allen Bernatowicz, an accountant, testified for Apotex that the company stood to lose up to $4 billion in costs, future sales and lost opportunity if it was forced to withdraw its drug from the marketplace.

He said Sanofi-Aventis and Bristol-Myers stood to lose half as much and could withstand the hit much easier because they were much larger than Apotex, a company with $1 billion in annual sales.