Hydrochlorothiazide is a medication primarily used to treat high blood pressure (hypertension) and edema (fluid retention) caused by various medical conditions such as heart failure, kidney disorders, or liver disease. It belongs to a class of drugs known as diuretics or "water pills," which help the body get rid of excess salt and water by increasing urine production. This helps lower blood pressure and decrease swelling in the body. Hydrochlorothiazide is often prescribed in combination with other blood pressure medications for better control of hypertension. It is usually taken by mouth in the form of a tablet.