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Alka-Seltzer. View of an Alka-Seltzer tablet effervescing as it dissolves in water. Alka-Seltzer is a non-prescription proprietary antacid. Each tablet contains sodium bicarbonate, citric acid and aspirin. The sodium bicarbonate is present as a fast-acting acid-neutralizing compound. The citric acid is used to promote effervescence of the tablets in solution, and to prevent 'acid rebound' (over-production of stomach acids following the dose of bicarbonate). Aspirin is included as a mild pain reliever, its speed of action is promoted by the carbon dioxide generated by the reaction of the bicarbonate with stomach acids and the tablet's citric acid. (KEYSTONE/SCIENCE PHOTO/Charles D. Winters) === ===