Hydrochlorothiazide is a medication used to treat high blood pressure and edema (fluid retention) caused by various medical problems, including heart, kidney, and liver disease, and to treat edema caused by using certain medications including estrogen and corticosteroids. It is a thiazide diuretic, which means it works by causing the kidneys to get rid of unneeded water and salt from the body into the urine. Hydrochlorothiazide may also be used to treat patients with diabetes insipidus and to prevent kidney stones in patients with high levels of calcium in their blood. It is available only with a doctors prescription.