Hydrochlorothiazide is a prototype drug of thiazide diuretics. Hydrochlorothiazide is antihypertensive agent. It increases the urination and reduces the amount of water and sodium retained by the body. The drugs in this class are formally called benzothiadiazide, usually shortened to thiazides. The nature of the hetrocyclic rings and the substitution on these ring may vary among the congeners but all of them retain a unsubstituted sulfonamide group. Development of thiazides and other modern diuretics began when a perceptive physician noticed that patients receiving sulfanilamide, an early antimicrobial agent, developed metabolic acidosis and a very alkaline urine. Careful study revealed that the drug was causing sodium bicarbonate diuresis. The realization that it could be useful diuretic led first to the development of acetazolamide and then to the thiazides.