About spironolactoneselective aldosterone antagonist, antiandrogen, potassium-sparing diuretic.mechanism of action of spironolactonespironolactone is a steroid with structural similarity to aldosterone which exerts it`s pharmacological actions by acting as a competitive antagonist of aldosterone- in the distal part of nephron (late distal tubule and collecting duct). it prevents potassium secretion and decreases sodium reabsorption. it binds with minerelocorticoid receptor from the interstitial side and inhibits formation of aldosterone induced proteins competitively and thus increases sodium excretion and decreases potassium excretion. it increases ca excretion also. it also blocks the effects of aldosterone on arteriolar smooth muscles and reduces peripheral resistance. it prevents aldosterone induced sodium and water retention and thus reduces blood volume and used as an effective diuretic in treatment of hypertension and also used to reduce potassium loss