astringent, any of a group of substances that cause the contraction or shrinkage of tissues and that dry up secretions. 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). Used in medicine to reduce swollen mucous membranes that result from inflammations of the nasal, gastrointestinal, and urinary passages, astringents are also frequently employed to dry up excessive secretions and (in this connection they are often known as styptics) to stop bleeding.