Antacids are medicinal substances, capable of neutralizing acidity of the blood, the secretions, etc., by combining with the acid and counteracting it.

The alkalies, alkaline earths, and their carbonates are the substances included in this class, and the former are more energetic in their action than the alkaline earths, such as magnesia. The carbonates of these earths have little or no chemical influence upon the tissues.

This class of remedies, in the form of alkaline preparations, are administered internally as antacids in cases where an unusual quantity of acid is generated in the stomach - a common symptom of dyspepsia, which is indicated by acid eructations, violent heartburn and marked effervescence when a carbonated alkali is taken; the acid in the stomach laying hold of the alkaline base, and resulting in a large quantity of carbonic acid being driven off.

Alkalies are also employed to relieve irritability of the stomach and check vomiting; also as antidotes in cases of poisoning from acids ; also as antilithics, to neutralize lithic acid when it is separated in unusual quantity by the urinary secretion; also as lithontriptics, or solvents of calculi, more particularly lithates ; also employed in cases of acute rheumatism and gout, to neutralize the excess of acid in the blood; also in diabetes mellitus, and to relieve irritability of the urinary organs, cutaneous irritation, itching of the anus, especially when such conditions are dependent on an excess of acid in the system; and also as antiplastics and resolvents in inflammation, and as diuretics. As a general rule the administration of antacids should be preceded by an emetic or cathartic, and in some cases both.

When a permanent effect is desired, antacids are usually combined with tonics and aromatics, and occasionally with narcotics ; for when given alone the relief afforded is but transient. To prevent an irritant and purgative action on the bowels, and also to facilitate their absorption, antacid preparations are administered in a state of large dilution.

In dental practice antacids are also employed for correcting acidity of the fluids of the mouth, often a result of acid eructations from the stomach. The class of antacids comprise the preparations of potassium, sodium, lithium, ammonium, magnesium and calcium. The antacids employed in dental practice are principally precipitated chalk, lime water, carbonate of soda, etc.