Hydrochlorothiazide is primarily used as a diuretic to treat high blood pressure and fluid retention caused by various conditions such as heart failure, kidney disorders, or liver disease. By increasing urine production, it helps reduce excess fluid in the body, thereby lowering blood pressure and alleviating swelling. It is often prescribed as part of a combination therapy with other antihypertensive medications for more effective blood pressure control. Additionally, it is sometimes used to prevent kidney stones in individuals with high calcium levels in their urine. Its ability to manage fluid balance and blood pressure makes it a key component in the treatment of cardiovascular and renal conditions.