What is Cozaar? Cozaar (losartan) is an angiotensin II receptor antagonist. Losartan keeps blood vessels from narrowing, which lowers blood pressure and improves blood flow. Cozaar is used to treat high blood pressure (hypertension) in adults and children who are at least 6 years old. It is also used to lower the risk of stroke in certain people with heart disease. Cozaar is used to slow long-term kidney damage in people with type 2 diabetes who also have high blood pressure.