I don't know what it is!! Maybe it's the sweetener, but whenever I drink any Zero soda, or any Max soda, especially the colas. I get heartburn so bad that it feels like someone has been punching me in the chest for 3 days. And it lasts for about 3 days. I've tested the theory a few times at great personal risk lol. Anyone else experience this or something similar??? I think the sweetener is ACE-K. I can drink diet sodas and regular sodas just fine. I also had no issue with the Coke C2 or Pepsi Edge sodas.

If it's just the colas that are causing problems, it may be that you're sensitive to phosphoric acid. Most other drinks (Dr Pepper is an exception) don't contain it.

Many other diet sodas contain ace-k as well. Pretty much anything from Coke, other than Diet Coke and CF Diet Coke has it. It helps extend the shelf life and cuts down on the artificial sweetener taste.