Guanadrel is a spiroketal formed by the formal condensation of the keto group of cyclohexanone with the hydroxy groups of 1-(2,3-dihydroxypropyl)guanidine. It is a postganglionic adrenergic blocking agent formerly used (as the sulfate salt) for hypertension management. It has been largely replaced by drugs less likely to cause orthostatic hypotension. It functions as an antihypertensive and adrenergic antagonist, and is a member of guanidines and spiroketals. It is the conjugate base of guanadrel(1+).