Historical Properties and Uses of Tannic acid

Because tannins are naturally astringent, almost all plants containing them are used as astringents. Tannic acids help eliminate diarrhea, reduce swelling of hemorrhoids, loosen catarrh in the respiratory system, and control various kinds of internal bleeding. Externally, tannins are beneficial in rubs for aching muscles and joints, in salves for open, slow-healing sores, and as antiseptics. Plants high in tannic acid have been used to treat cancers, but tannic acid itself has been found to be carcinogenic under certain conditions.
In very small amounts, tannic acid is approved for food use and is added to many commercial foods. Since the astringecy of tannic acid involves the precipitation of protein, the addition of protein to a tannin-rich tea would "bind" the tannin, rendering it biologically inert.