TUMS is an antacid used to relieve heartburn, sour stomach, acid indigestion, and upset stomach associated with these symptoms. TUMS starts to neutralize the heartburn-causing acid in your stomach on contact which provides fast relief for your tough heartburn. Many other heartburn medications are absorbed into the bloodstream and can take hours or day to fully work. TUMS, with active ingredient Calcium Carbonate, travels directly to the source of the heartburn.