The decision to swap sweeteners comes as Americans keep turning away from popular diet sodas. Rival Coca-Cola said this week that sales volume for Diet Coke, which also uses aspartame, fell 5 per cent in North America in the first three months of the year.
Atlanta-based Coca-Cola said in a statement that it has no plans to change the sweetener in Diet Coke, which is the country's top-selling diet cola.
The Food and Drug Administration says aspartame, known by the brand names Equal and NutraSweet, is "one of the most exhaustively studied substances in the human food supply, with more than 100 studies supporting its safety."
Executives at Coke and Pepsi blame the declines on perceptions that the sweetener isn't safe.
John Sicher, publisher of industry tracker Beverage Digest, noted that attitudes about aspartame can be very negative. Using an online tool called Topsy that measures Twitter sentiment on a scale of 0 to 100, he noted "aspartame" got a 22 ranking, below a 38 ranking for "Congress."