Astringents are medicines which cause the contraction of living tissues, diminishing the amount of blood or other fluid in them, and reducing hemorrhage, or, through their constipating action, limiting the intestinal secretions as well as those from mucous membranes generally.

They act chemically upon the tissues, and when taken internally, their influence is similar to that of tonics, their principal use being in cases of relaxed conditions of the muscles and fibers or of the mucous membranes characterized by excessive secretion.

Astringents are more or less irritating, and should therefore not be employed, as a rule, in acute inflammatory conditions. There are, however, four exceptions - lead acetate or subacetate, bismuth subnitrate or subcarbonate, cerium oxalate, and silver nitrate - which are sedative astringents and would be indicated in acute inflammatory states.

Astringents are divisible into (1) mineral astringents; (2) vegetable astringents.

A large number of the heavy metals, while they have a profound influence on the nutritive processes, are not known to possess any beneficial action, but rather so depress metabolism as to be more harmful than useful. They are employed, however, very widely for their local astringent and antiseptic effects.

They are silver, copper, lead, zinc, aluminum, cerium, and bismuth.

The astringent and escharotic actions of all of the metals are prominent. Such actions depend, however, in the dissociable salts, upon both ions of the compound, the acid as well as the metal. From the standpoint of the metal they vary in strength of action widely; thus, the series, from strongest to weakest, is: mercury, tin, silver, copper, zinc, iron, aluminum, and lead. For the acid ion, from most to least active, hydrochloric, nitric, sulphuric, oxalic, tartaric, citric, acetic acids. Thus, a combination of an active metallic ion, such as Hg, with an active acid, C1, produces the very corrosive HgCl2, corrosive sublimate, while lead acetate, the combination of the two mildest metal and acid ions respectively, produces a mild astringent salt. The chlorides of the heavy metals are usually soluble in water. Should a chloride be insoluble in water, it will not act as a caustic - as, for example, the insoluble, and consequently inert, silver chloride.

By a proper regulation, therefore, of acid and metal, almost any grade of action may be brought about. Thus, for definite dilutions and concentrations those (1) Compounds that are mainly astringent are: alum, lead acetate, zinc oxide, bismuth subnitrate, etc.

(2) Compounds that are astringent and caustic are: iron salts, zinc sulphate, zinc acetate, copper acetate, silver nitrate, lead nitrate, etc.

(3) Compounds which are mainly corrosive and caustic are: all mercury salts, zinc chloride, tin chloride, antimony chloride, copper sulphate, etc. Certain drugs which in a concentrated state are caustic are, if sufficiently diluted, astringent, as is the case with sulphuric acid.

An astringent drug employed to check hemorrhage is called a styptic, as, for example, the subsulphate of iron.

Certain salts of iron are powerfully astringent, although classed with iron under the Restoratives. Diluted sulphuric and nitric acids also possess marked astringent properties. (See Mineral Acids.)

Inasmuch as many of these metals when taken for some time cause symptoms of chronic poisoning, they are here considered in greater detail.