antacid, any one of several basic substances that counteract stomach acidity (see stomach). Antacids are used by physicians to treat hyperchlorhydria, i.e., the excessive production of hydrochloric acid by the parietal cells lining the stomach. Commonly used antacid preparations, most sold without prescription, contain sodium bicarbonate, magnesium hydroxide, or aluminum hydroxide. Some preparations contain substances such as magnesium trisilicate that reduce the formation of gas.