(KLOH-nih-deen HY-droh-KLOR-ide)A drug used to treat high blood pressure. It is also being studied in the treatment of certain types of cancer pain and as an aid to stop smoking. It blocks the release of chemicals from nerve endings that make blood vessels constrict (get narrower). Clonidine hydrochloride is a type of antihypertensive agent and a type of alpha-adrenergic agonist. Also called Catapres.