Ismelin, guanethidine monosulfate USP, is an antihypertensive, available as tablets of 10 mg and 25 mg for oral administration. Each 10-mg and 25-mg tablet contains guanethidine monosulfate USP equivalent to 10 mg and 25 mg of guanethidine sulfate USP. Its chemical name is [2-(hexahydro-1(2H)-azocinyl)ethyl] guanidine sulfate 1:1.