Hyzaar (losartan potassium-hydrochlorothiazide) is a combination of an angiotensin II receptor antagonist and a thiazide diuretic (water pill) used to treat high blood pressure (hypertension).It is also used to lower the risk of stroke in certain people with heart disease. Hyzaar is available in generic form.ast reviewed on RxList 6/9/2015. Hyzaar (losartan potassium-hydrochlorothiazide) is a combination of an angiotensin II receptor antagonist and a thiazide diuretic (water pill) used to treat high blood pressure (hypertension). It is also used to lower the risk of stroke in certain people with heart disease.