NEW YORK (MarketWatch) -- American Express Co.’s stock dropped to a four-month low Thursday, after the charge-card company said the loss of its exclusivity deal with Costco Wholesale will hurt profit and sales this year and in 2016.

The stock AXP, -1.16% , a component of the Dow Jones Industrial Average DJIA, -0.87% , was down as much as 7% in intraday trade, before closing down $5.53, or $6.43%, at $80.48. That’s the lowest close since Oct. 16. The stock’s decline reduced the Dow’s gain by about 36 points.