The generic drug maker Apotex yesterday began shipping a cheaper form of Plavix, the anticlotting agent that is one of the world’s best-selling drugs, in an embarrassing setback for the drug’s brand-name marketers, Sanofi-Aventis and Bristol-Myers Squibb.

A year ago, analysts had predicted that the companies had a better than 60 percent chance of winning a patent dispute with Apotex. But an attempt by the companies to settle the dispute has backfired, opening the way for Apotex to enter the market.

“It looks like a much smaller generic private company completely outmaneuvered two of the giants of the pharmaceutical industry,” said Gbola Amusa, European pharmaceutical analyst for Sanford C. Bernstein & Company. “It’s not clear how or why that happened. The reaction from investors and analysts has ranged from shock to outright anger.”

The generic version, called clopidogrel, is expected to be available in the United States beginning today at prices about 30 percent below the $4-a-day retail price for Plavix. The drug, used for preventing heart attacks and strokes, had sales of more than $6 billion last year.

Shares of Bristol-Myers, which gets nearly one-fifth of its revenue from Plavix, were down 6.85 percent yesterday on the prospect of a major erosion of those sales. The American depository receipts of Sanofi, a French company that sells Plavix outside the United States, were off by nearly 1 percent.

Barry Sherman, Apotex’s chief executive, predicted that clopidogrel, the biggest-selling drug ever to go generic, would mark the “largest and most successful launch” of a generic drug in history. Apotex’s move into the market follows the government’s recent rejection of the proposed patent settlement, which Bristol-Myers and Sanofi had hoped would keep the generic drug off the market until 2011.

Despite the government’s rejection of the deal, some terms of that agreement remain in effect. And they hold at least two significant disadvantages for Bristol and Sanofi. Under the terms, the companies must wait five business days before seeking a federal injunction against Apotex’s shipments, giving the generic company an opportunity to potentially flood the market with its generic drug before a court can step in.

The big companies also negotiated away their rights under federal law to seek triple financial damages if they eventually win the patent dispute in court. That proviso removed one of the major deterrents to a generic competitor’s entering the market while a drug is still under patent.

Analysts said the developments raised doubts about the leadership of Bristol-Myers and Sanofi and the wisdom of the concessions they had made to Apotex. And because the abortive patent settlement is also now the subject of a federal criminal inquiry, some analysts raised questions about whether Peter R. Dolan, Bristol-Myers’s chief executive, can survive.

“On the surface, it doesn’t look good,” said Jami Rubin, a Morgan Stanley analyst. “Credibility, I think, has been severely set back.”

In a letter to employees yesterday, Mr. Dolan warned that there would be negative news accounts of the generic introduction, but he defended the company’s efforts to protect its intellectual property rights.

A Bristol-Myers spokesman, Tony Plohoros, said the company was evaluating its legal and commercial options. Those could include a decision by the company to drastically lower the price of Plavix or to introduce its own generic equivalent.

In a statement, the French company Sanofi said it was also evaluating possible remedies against Apotex, which a lawyer for Bristol-Myers accused in court of intentionally sabotaging the deal.

Mr. Sherman, in a telephone interview, all but ridiculed his two big rivals, saying they had naïvely agreed to conditions that allowed his company to bring its product to market even though the deal was rejected by regulators.

“I think they acted foolishly in a number of ways,” said Mr. Sherman, a Toronto billionaire who amassed his fortune in the generic drug business.

Mr. Sherman said that he had never expected the American government to approve the deal, but that he had conducted the negotiations in a way to let him push the Apotex drug onto the market.

Mr. Sherman said Apotex was engaged in an “all-out launch” and has already shipped most of its inventory while manufacturing continues.