An astringent (also spelled adstringent) substance is a chemical compound that tends to shrink or constrict body tissues, usually locally after topical medicinal application. The word "astringent" derives from Latin adstringere, meaning "to bind fast".
Astringents are usually classified into three groups according to their mode of action:
(1) those that decrease the blood supply by narrowing the small blood vessels (e.g., epinephrine and cocaine),
(2) those that abstract water from the tissue (e.g., glycerol and alcohol), and
(3) those that coagulate the superficial tissue layers into a crust (e.g., metallic astringents, such as calamine or alum)
Astringent medicines cause shrinkage of mucous membranes or exposed tissues and are often used internally to check discharge of blood serum or mucous secretions like
sore throat,(ferric chloride astringent used)
mouth ulcers,(glycerol astringent used)
Externally applied astringents, which cause mild coagulation of skin proteins, dry, harden, and protect the skin.