Any substance, such as sodium bicarbonate, magnesium hydroxide, or aluminum hydroxide, used to relieve the discomfort caused by indigestion, gastritis, and several forms of ulcers. Antacids counteract or neutralize gastric acidity for up to three hours after a single dose. Antacids should be taken when gastric acidity is most likely to be increasing—namely, between one and three hours after each meal and at bedtime.

This entry comes from Encyclopædia Britannica Concise.
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