This from the British Pharmacuetical codex (recent)

Action and Uses.—Pyrogallic acid is a powerful antiseptic by reason of its affinity for oxygen. Taken internally it exerts a toxic action on the blood, methaemoglobin is formed, passes into the plasma, and leaves the red corpuscles as granular debris; jaundice and acute nephritis may follow, hence the drug is now very rarely given internally. It is used as an antiseptic and mildly irritant ointment in chronic skin diseases such as psoriasis, also to destroy lupus; it is of value as a parasiticide in ringworm. Mixed with four times its weight of starch, the acid is applied as a powder to phagedaenic chancres. It has the disadvantage of staining the skin and hair black, and must be used with caution on account of the danger of absorption. Stains upon the skin may be removed with ammonium persulphate. Pyrogallic acid is an ingredient of hair dyes, usually with silver nitrate; and is largely used as a reducing agent in photography.