This substance is prepared from nutgalls. It is slowly but completely soluble in less than its own weight of water or alcohol, and, with the aid of heat, in its own weight of glycerin. It is used locally in 5 to 20 per cent. preparations, or internally in dose of 5 grains (0.3 gm.). The ointment and the glycerite are of 20 per cent. strength. The troches contain 1 grain (0.06 gm.) in each. Tannic acid is incompatible with alkaloidal salts, metallic salts (such as mercuric chloride), lime-water, gelatin, and protein. The precipitation of the gelatin and proteins of hides is "tanning," and changes the hides into leather. In like manner tannic acid renders insoluble the coatings of gelatin capsules and pills.

Its astringency depends upon its power to precipitate the proteins of the superficial cells, thus causing shrinking of the tissues and stoppage of secretion. It checks small hemorrhages, i. e., is hemostatic or styptic, by coagulating the blood. In the stomach it precipitates the proteins of the food, but these re-dissolve in the gastric juice. Its effect on mucous membranes is to check secretion. Strasburger believes that the lessening of intestinal mucus by astringents results in a great diminution in the number of bacteria in the feces. In the intestines free tannic acid is constipating, but it soon changes to sodium tannate and then to sodium gallate, which is not astringent. It is absorbed and excreted as sodium gallate, and has no astringent or styptic power after absorption. Because of the rapid disappearance of tannic acid from the intestines, preparations of the vegetable drugs are preferred in diarrhea, the colloid and other extractive vegetable matters tending to retard the chemic changes and absorption of the tannic acid. If in too concentrated form, tannic acid is an irritant.

Therapeutics

1. To harden the skin, as in threatened bedsore.

2. As a gargle or swab in relaxed sore throat.

3. As a hemostatic for small accessible hemorrhages.

4. As chemic antidote in poisoning by alkaloidal and metallic salts, especially those of antimony, with which it forms a very insoluble substance.

5. In the form of suppository, each containing 5 grains (0.3 gm.), in prolapse of the rectum or bleeding internal hemorrhoids.

6. In diarrhea - the vegetable astringents.

Tannigen (diacetyltannin), tannoform (formaldehyde-tannin), tannopin (hexamethylenamine-tannin), and tannalbin (egg-albumin tannate) are all compounds marketed for diarrhea. The claims are made for them that they do not act in the stomach, but liberate the tannic acid in the intestines. Dose of each, 10 grains (0.7 gm.).

Styptics

The astringent remedies especially used as styptics, that is, to check hemorrhage, are: Solutions of epinephrine, antipyrine, alum, silver nitrate, ferric chloride, ferric sulphate, and ferric subsulphate (Monsell's solution), very hot water, very cold water, glycerite of tannic acid, and 2 per cent. acetic acid.