Bristol-Myers Squibb Co. shares tumbled after the company pushed back its forecast for when a new crop of drugs will reach $10 billion in sales, raising investor concern about how soon the company can recoup lost ground after one of its top drugs went off patent.

The shares lost as much as 5.2% at Thursday’s New York market open, the most intraday in a year. They had fallen 21% this year through Wednesday’s close.

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