Losartan, sold under the brand name Cozaar among others, is a medication used to treat high blood pressure (hypertension). It is in the angiotensin receptor blocker (ARB) family of medication, and is considered protective of the kidneys. Besides hypertension, it is also used in diabetic kidney disease, heart failure, and left ventricular enlargement. It comes as a tablet that is taken by mouth. It may be used alone or in addition to other blood pressure medication. Up to six weeks may be required for the full effects to occur.

Common adverse effects include muscle cramps, stuffy nose, dizziness, cough, high blood potassium, and anemia. Severe adverse effects may include angioedema, low blood pressure, and kidney problems. Use during pregnancy may result in harm to the baby. Use is not recommended during breastfeeding. It works by blocking angiotensin II.

Losartan was patented in 1986, and approved for medical use in the United States in 1995. It is on the World Health Organization's List of Essential Medicines. It is available as a generic medication. In 2023, it was the eighth most commonly prescribed medication in the United States, with more than 56 million prescriptions. A version combined with hydrochlorothiazide is available which, in 2023, was the 65th most commonly prescribed medication in the United States, with more than 9 million prescriptions.

Losartan potassium is chemically described as 2-butyl-4-chloro-1-[p-(o-1H-tetrazol-5-ylphenyl)benzyl]imidazole-5-methanol monopotassium salt. Its empirical formula is C₂₂H₂₃ClN₆O , and its molecular weight is 422.9.

Losartan is generally marketed as the (basic) potassium salt of the aromatized negatively charged tetrazole, called "losartan potassium". The molecule has an extended biphenyl group with a tetrazole which is being used in place of the carboxylic acid as a bioisostere.

Losartan is used for hypertension, including in people with left ventricular hypertrophy (enlarged heart muscle), and kidney dysfunction among type II diabetics. It may also delay progression of diabetic nephropathy. It is a suitable pharmacological agent for the reduction of kidney (renal) disease progression in patients with type 2 diabetes, hypertension, and microalbuminuria (>30 mg/24 hours) or proteinuria (>900 mg/24 hours).

Although evidence shows calcium channel blockers and thiazide-type diuretics are preferred first-line treatments for most people (due to both efficacy and cost), an angiotensin II receptor antagonist such as losartan is recommended as first-line treatment in people under the age of 55 who cannot tolerate an ACE inhibitor. One study demonstrated losartan was superior to atenolol in the primary prevention of adverse cardiovascular events (myocardial infarction or stroke), with a reduction in cardiovascular morbidity and mortality for a comparable reduction in blood pressure. The maximal effects on blood pressure usually occur within 3–6 weeks of starting losartan.

The most common adverse effects for losartan in adults are upper respiratory infections, dizziness, and back pain. People with type 2 diabetes and kidney disease may experience diarrhea, fatigue, low blood pressure, low blood glucose, elevated potassium, chest pain, or allergic reaction. Losartan should not be taken by people who are diabetic and taking aliskiren. Anemia may occur, due to inhibition of the renin–angiotensin system. As with other angiotensin receptor blockers, losartan may injure the liver, although this effect appears to be rare. Electrolyte imbalances may occur in people with kidney problems who take losartan. Adverse outcomes do not differ by sex, age, or race.