Coca-Cola Co. will introduce an energy drink called "Relentless" in the U.S. in September, according to trade journal Beverage Digest. The release will be aimed at territories where Coca-Cola bottlers do not carry Monster energy drinks. The Coca-Cola system is already selling Relentless in Europe, and touting the brand's increasing sales. The U.S. energy drink business posted slim growth in the first quarter, according to Beverage Digest, with Monster, Red Bull, Rockstar and NOS growing. But Coke's Full Throttle brand declined.