Losartan potassium is a medication used to treat high blood pressure (hypertension) in adults and children over 6 years old. It is also used to help protect the kidneys from damage due to diabetes and to decrease the risk of stroke in people who have high blood pressure and a heart condition called left ventricular hypertrophy. Losartan potassium is an angiotensin II receptor blocker (ARB) that works by blocking a substance in the body that causes blood vessels to tighten, allowing the blood to flow more smoothly and the heart to pump more efficiently. It is important to take losartan potassium as prescribed by a doctor, as it comes with serious risks if not taken properly.