An organic compound having the property of lowering blood pressure in animals and humans. Among the better-known types are the alkaloid reserpine and its derivative syrosingopine, guanethidine sulfate, α-methyl dopa (α-methyl-3,4-dihydroxyphenylalanine), and hydralazine. They function by various nerve-blocking mechanisms involving structural antagonism. They should be taken only by prescription.