Bromo-Seltzer is a brand of antacid formulated to relieve pain occurring together with heartburn, upset stomach, or acid indigestion. It originally contained sodium bromide and acetanilide, both toxic substances which were eventually removed. Its current formulation contains the pain reliever aspirin and two reactive chemicals – sodium bicarbonate and citric acid – which creates effervescence when mixed with water. Sodium bicarbonate is an antacid.