Hydrochlorothiazide is a thiazide diuretic (water pill) that helps prevent your body from absorbing too much salt, which can cause fluid retention. Losartan is in a group of drugs called angiotensin II receptor antagonists. Losartan keeps blood vessels from narrowing, which lowers blood pressure improves blood flow. The combination of hydrochlorothiazide and losartan is used to treat high blood pressure (hypertension) and to reduce the risk of stroke in people with hypertension.