Bristol-Myers Squibb Co. said yesterday it is the subject of a criminal antitrust probe over a pending deal with a generic drug maker involving its bestselling product, Plavix, and reported a 33% drop in second-quarter profit.

Bristol-Myers’ shares fell nearly 8% after it announced the U.S. Department of Justice had launched an investigation into a settlement awaiting regulatory approval that it had reached with Apotex Inc. to keep a generic version of blood-thinner Plavix off the market until at least 2011.

Analysts said the Justice Department inquiry casts further doubt over the future of Bristol-Myers’ best-selling drug just when the market is awaiting the outcome, as early as Friday, of an investigation by states’ attorneys general into the same deal.

The states’ attorneys general and Federal Trade Commission can reject deals that Bristol-Myers reaches with generic companies over patents as a result of an agreement reached with the company regarding earlier settlements.

Last March, Bristol-Myers and marketing partner Sanofi-Aventis SA agreed to pay Apotex at least $40 million in a deal under which the generic company ended its Plavix patent challenge. The agreement sparked numerous lawsuits that alleged consumers were being denied access to cheap generic drugs.

Sanofi is also under investigation by the justice department.