Losartan is an angiotensin II receptor blocker (ARB). It works by blocking a substance in the body that causes blood vessels to tighten. As a result, losartan relaxes the blood vessels. A lower blood pressure will increase the supply of the blood and oxygen to the heart. Losartan is used to treat high blood pressure and to help protect the kidneys from damage due to diabetes. Learn about side effects, drug interactions, dosages, warnings, and more. Losartan (Cozaar) is an angiotensin II receptor blocker (ARB) commonly prescribed to treat high blood pressure. Common losartan side effects include dizziness, respiratory infections, and back or joint pain. Fatigue, headache, and digestive issues are also possible. NHS information about the medicine losartan, including what it's used for, who can use it, how to use it and side effects. Overview: Losartan is used for high blood pressure or in certain people with high blood pressure or diabetes. Common side effects may include dizziness, stuffy nose, or back pain. Do not drive or. Losartan is in a class of medications called angiotensin II receptor antagonists. It works by blocking the action of certain natural substances that tighten the blood vessels, allowing the blood to flow more smoothly and the heart to pump more efficiently. Losartan, sold under the brand name Cozaar among others, is a medication used to treat high blood pressure (hypertension). [4] It is in the angiotensin receptor blocker (ARB) family of medication, and is considered protective of the kidneys. Losartan is a prescription drug to treat high blood pressure, diabetic nephropathy, and stroke risk. Learn about side effects, warnings, dosage, and more. Losartan starts lowering blood pressure within about a week, and its full effect can take 3 to 6 weeks. Dizziness or lightheadedness is more likely during this time. Some side effects, like high potassium or changes in kidney function, often cause no symptoms at first. 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.