When I drink a single can of Diet Coke or anything with aspartame, I get crippling stomach aches and then sudden diarrhea, all within about 2 hrs and very predictable. It's definitely not harmless. This doesn't happen to me with stevia or sucralose, and I know sucralose isn't good for your either.
Coke Zero is sweetened with aspartame and phosphoric acid. It's just formulated differently to avoid the taste associated with Diet Coke, which some find unpleasant.
> Coca-Cola updated the recipe for Coca-Cola Zero Sugar in the US, quietly adding stevia to the existing blend of aspartame and acesulfame potassium (Ace-K) in late 2023/early 2024