Diet Coke
Diet Coke does not use a modified form of the Coca-Cola recipe, but instead an entirely different formula. The controversial New Coke, introduced in 1985, used a version of the Diet Coke recipe that contained high fructose corn syrup and had a slightly different balance of ingredients. In 2004, Coca-Cola introduced Coca-Cola C2, which it claims tastes much closer to Coca-Cola but contains half the carbohydrates. In 2005, the company introduced Coca-Cola Zero (replaced by "Coca-Cola Zero Sugar" in 2017), a sugar-free variation of regular Coca-Cola.
Diet Pepsi
Diet Pepsi competed primarily with The Coca-Cola Company's Tab in the 1960s and 1970s; however, The Coca-Cola Company introduced Diet Coke in 1982 which has since been the principal competing product to Diet Pepsi. As of 2010, Diet Pepsi represented a 5.3 percent share of all carbonated soft drink sales in the United States, and was ranked as the #7 soft drink brand by volume. In the same year, Diet Coke was recorded as having a 9.9 percent market share.