This is the astringent principle of oak bark, of nut galls, and of many other vegetable materials. Its presence in tea-leaves accounts for iron spoons being blackened when left in tea. Catechu and other vegetable astringent medicines contain tannic acid, some of them also the very similar gallic acid.

Tannin is often given as a medicine in pill for diarrhæa and for hemorrhages. A good astringent pill is made with three grains of tannin and a little opium, from one-twelfth to one-half a grain of the latter, according to the case.

Tannin is also frequently made part of an astringent gargle, particularly in rather chronic (prolonged) cases of sore throat.