What is Losartan? Losartan, sold under the brand name Cozaar, is used to treat high blood pressure (hypertension). It’s also used to lower the risk of stroke in some patients with heart disease. Losartan is an angiotensin receptor blocker (ARB) that blocks a substance that causes blood vessels to tighten. By relaxing blood vessels, Losartan helps a person maintain a lower blood pressure, increasing the supply of oxygen and blood to the heart.