Diet Coke may be the big loser in the cola wars.
Sales of the soda plunged eight percent around the globe during the third quarter of this year.
That follows steep declines for Diet Coke in the first half of this year.
Diet drinks have struggled amid concerns that their artificial ingredients make them less healthy than advertised.
Pepsi responded by removing the sweetener Aspartame from Diet Pepsi.
But Coke continues to use the sweetener in Diet Coke.
Diet Coke also may be struggling due to the growing popularity of no-calorie Coke Zero.
That soda saw volume grow eight percent in the third quarter.