Hydrochlorothiazide is a diuretic medication often used to treat high blood pressure and swelling due to fluid build up. Uses · How to use Hydrochlorothiazide · Related Links. Hydrochlorothiazide is the most commonly prescribed thiazide diuretic. Hydrochlorothiazide is used to treat edema (fluid retention; excess fluid held in body tissues) caused by various medical problems, including heart, kidney, and. Hydrochlorothiazide is a thiazide diuretic (water pill) that helps prevent your body from absorbing too much salt, which can cause fluid retention. Hydrochlorothiazide is also used to treat fluid retention (edema) that is caused by congestive heart failure, severe liver disease (cirrhosis), kidney disease, or treatment with a steroid or hormone medicine.