The therapeutically employed antacids are certain salts of the alkalies, potassium, sodium, lithium, and ammonium, and certain salts of the alkaline earths, magnesium and calcium. Of the metals mentioned, K, Na, and Li are ions of ready absorbability from the alimentary tract, while Mg and Ca are absorbed with comparative difficulty. Hence after a local action in the stomach the salts of the former for the most part manifest a systemic action, while those of the latter have a special intestinal activity, magnesium salts being laxative and those of calcium constipating.

The antacids are of two types: I. Those of alkaline reaction.

II. Those not of alkaline reaction.

The Antacids Of Alkaline Reaction

These can neutralize acids, and they have both a local and a systemic effect as alkalinizers. They are chiefly oxides, hydroxides, and carbonates, and may be differentiated into two groups, the caustic alkalies and the mild alkalies.

(a) The caustic alkalies are the hydroxides of potassium (Koh) and sodium (NaOH) and the oxide of calcium (CaO, lime; Lat, calx). They destroy tissue by abstracting water, by dissolving albumin, and by saponifying fats. Even in dilute solution the potassium and sodium hydroxides are more penetrating and more irritant than the other alkalies. The official solutions of potassium hydroxide and sodium hydroxide are of about 5 per cent. strength. They are strongly caustic.

(b) The milder alkalies are the carbonates and bicarbonates of potassium, sodium, and lithium, and the carbonates and hydroxides of magnesium and calcium. The salts of potassium, sodium, and lithium are preferred for simple alkalinity, the magnesium salts when there is constipation, and the calcium salts when there is diarrhea.