Losartan is in a group of drugs called angiotensin II receptor antagonists. Losartan keeps blood vessels from narrowing, which lowers blood pressure improves blood flow. Losartan is used to treat high blood pressure (hypertension) and to rCozaar tablets contain the active ingredient losartan potassium, which is a type of medicine called an angiotensin II antagonist. It works by preventing the action of a hormone in the body called angiotensin II. Angiotensin II normally acts on special receptors in the body, with two main results. Firstly, it causes the peripheral blood vessels to narrow, and secondly, it stimulates the production of another hormone called aldosterone. Aldosterone causes salt and water to be retained by the kidneys, which increases the volume of fluid in the blood vessels. Losartan blocks the receptors that angiotensin II acts on, and so prevents its actions. The main result of this is that the peripheral blood vessels are allowed to widen, which means that there is more space and less resistance in these blood vessels. This is the main mechanism by which the pressure in the blood vessels is lowered. Blocking the actions of angiotensin II also reduces the action of aldosterone on the kidneys. The result of this is an increase in the amount of fluid removed from the blood by the kidneys. This decreases the amount of fluid in the blood vessels, which also lessens the resistance and pressure in the blood vessels. The combined overall effect of these changes is to lower the blood pressure, hence losartan is used to treat high blood pressure.