Alka-Seltzer is an over-the-counter antacid and pain relief medication that contains acetylsalicylic acid (aspirin), citric acid, and sodium bicarbonate. It is typically used to relieve heartburn, acid indigestion, upset stomach, and headaches. When dissolved in water, it produces effervescence, which helps to deliver the active ingredients more quickly. Alka-Seltzer is commonly available in various formulations, including versions specifically designed for cold and flu symptoms.