A substance that neutralizes excess stomach acid, relieving the burning of heartburn and acid indigestion. Common antacids include calcium carbonate, magnesium hydroxide, and aluminum hydroxide, which react directly with hydrochloric acid in the stomach to raise pH within minutes. They differ from acid-suppressing drugs, which reduce acid production over hours: antacids act fast but briefly, making them ideal for occasional symptoms rather than chronic disease. Formulations balance the constipating effect of aluminum against the laxative effect of magnesium, and add flavorings to make the chalky suspensions palatable. While generally safe for short-term use, overuse can disturb mineral balance and mask conditions needing proper diagnosis. Dietary strategies that reduce reflux frequency remain the first line, with antacids as the backup.