Tannin has a bitter astringent taste, and constringes the mucous membrane. In the stomach it enters into combination with albumen, and with the pepsin of the gastric juice, which it precipitates from its solution. Tannin, therefore, impairs digestive power by rendering the pepsin inoperative. It diminishes secretion of the mucous membrane by virtue of its power to contract the caliber of the vessels, and it restrains peristalsis by its action on the muscular layer; hence the constipating effects which follow its use. If long continued in considerable quantity, tannin disorders digestion, sets up irritation of the mucous membrane, and gives rise to a febrile state and to wasting of the tissues.

Having such affinity for and coagulating action on albumen, it is obvious that tannin must diffuse into the blood with difficulty. A part undergoes conversion into gallic and pyrogallic acids in the stomach, and in this form is absorbed. Injected into the veins, tannic acid coagulates albumen, and the results which follow are due to multiple embolisms. Elimination of tannin takes place by the intestinal canal and by the kidneys, in the form of gallic and pyrogallic acids.