Hydrochlorothiazide (brand names Microzide and Oretic) is a popular option to treat high blood pressure. It is a thiazide diuretic, a.k.a. a “water pill,” that helps you get rid of excess salt and water. Hydrochlorothiazide, often called HCTZ, acts on the kidney filtration system. It prevents the kidneys from reabsorbing sodium and chloride ions, allowing them to be removed in the urine. Water typically follows ions—the more ions that get excreted, the more water goes with them. Losing this fluid can help lower blood pressure and improve swelling. Here’s more on what you need to know about hydrochlorothiazide, its primary uses, and what side effects to be aware of before taking the drug.