Shares of Bristol-Myers Squibb Co. sank more than 6 percent Tuesday as a generic drug maker disclosed it has begun selling a cheaper version of the big pharmaceutical company’s best-selling drug Plavix.

Apotex Corp. disclosed in a news release that it had launched a generic version of the blood thinner only hours after Bristol-Myers said in a filing with U.S. regulators that it had expected to see the competing drug in the market soon.