Plavix is an anti-blood clotting drug that is prescribed for many conditions. Its manufacturer Bristol-Myers Squibb had Plavix sales of $5.9 billion in 2005, second only to Pfizer’s Lipitor which also lost its patent protection in June.

Apotex, a Canadian drug manufacturer, was the first company to received FDA approval of a generic version of Plavix. Apotex also was engaged in a legal battle with Bristol Meyers and its co-marketer Sanofi-Aventis over the validity of the Plavix patent. Last month, several state attorneys general successfully challenged the validity of a proposed settlement of that case as anti-competititive, and the U.S. Justice Department is now investigating that settlement.

Apotex announced today that it is launching its generic Plavix, and Express Scripts, one of the big three prescription benefit managers, announced that it plans to begin dispensing that generic drug tomorrow. Apotex reportedly has five days to sell the generic drug before Bristol Myers and Sanofi-Aventis can apply for a court order blocking the sale on patent infringement grounds.