Hyzaar is a medication that combines losartan potassium, an angiotensin II receptor antagonist, and hydrochlorothiazide, a thiazide diuretic commonly referred to as a "water pill." It is used to manage hypertension (high blood pressure) and to reduce the risk of stroke in individuals with heart disease. The generic form of Hyzaar is also available.Hyzaar is composed of losartan and hydrochlorothiazide. Hydrochlorothiazide is a thiazide diuretic or "water pill" that prevents excessive salt absorption, thus preventing fluid retention.Losartan is an angiotensin II receptor antagonist, commonly known as an ARB blocker. It works by relaxing blood vessels, preventing their narrowing, and promoting better blood flow while simultaneously lowering blood pressure.