Losartan is an angiotensin II receptor antagonist. Losartan keeps blood vessels from narrowing, which lowers blood pressure and improves blood flow. Losartan is used to treat high blood pressure (hypertension). It is also used to lower the risk of stroke in certain people with heart disease. Losartan is used to slow long-term kidney damage in people with type 2 diabetes who also have high blood pressure. Losartan may also be used for purposes not listed in this medication guide.