What Is Losartan (Cozaar)? Uses, Side Effects & Warnings

Losartan, sold under the brand name Cozaar (now widely available as generic), belongs to the angiotensin II receptor blocker (ARB) drug class. It was the first ARB approved by the FDA and remains among the most prescribed blood pressure medications in the United States. Losartan tablets are available in 25 mg, 50 mg, and 100 mg strengths and is taken once or twice daily. It works by blocking a key hormonal system that raises blood pressure and stresses the kidneys and heart.

What Losartan Is Used For

Losartan has three FDA-approved indications. First, it treats hypertension (high blood pressure) in adults and children aged 6 and older. Second, it reduces the risk of stroke in patients with hypertension and left ventricular hypertrophy (an enlarged left heart chamber). Third, it slows the progression of diabetic nephropathy — kidney disease caused by type 2 diabetes — particularly in patients who also have elevated protein in their urine (proteinuria), which signals kidney stress.

Off-label, losartan is used in heart failure, particularly as an alternative for patients who cannot tolerate ACE inhibitors. It is sometimes used to slow the progression of Marfan syndrome-related aortic enlargement, and for certain other cardiovascular protective indications where blocking the renin-angiotensin-aldosterone system (RAAS) is beneficial.

How Losartan Works

The renin-angiotensin-aldosterone system (RAAS) is a hormonal cascade that regulates blood pressure and fluid balance. A key molecule in this cascade is angiotensin II, which binds to AT1 receptors on blood vessels and the heart. When angiotensin II activates these receptors, blood vessels constrict, blood pressure rises, the heart works harder, and the kidneys retain sodium and water. Losartan works by directly blocking AT1 receptors, preventing angiotensin II from binding and triggering these effects.

ACE inhibitors like lisinopril also target this system, but at a different point — they block the enzyme that converts angiotensin I into angiotensin II. While both drug classes ultimately reduce the effect of angiotensin II on blood vessels, the different mechanism of action has important consequences. ACE inhibitors allow bradykinin (another molecule) to accumulate, which causes a persistent dry cough in up to 20% of patients. ARBs like losartan do not affect bradykinin metabolism, so they do not cause this cough — which is why they are often prescribed as a cough-free alternative to ACE inhibitors.

Common Side Effects

• Dizziness or lightheadedness — especially on standing up (orthostatic hypotension), particularly when starting treatment
• Fatigue
• Upper respiratory infections
• Back pain
• Hyperkalemia (high potassium) — particularly in patients with kidney disease or those on potassium-sparing diuretics
• Elevated creatinine — some rise in kidney markers is expected and generally acceptable, but a large rise may require dose adjustment
• Low blood pressure

Notably absent: the dry, persistent cough seen with ACE inhibitors. This is the primary tolerability advantage of ARBs.

Important Warnings

Losartan is contraindicated in combination with aliskiren (a renin inhibitor) in patients with diabetes due to increased risks of renal impairment, low blood pressure, and hyperkalemia. Patients with bilateral renal artery stenosis can experience acute kidney injury when given RAAS-blocking drugs. Angioedema (swelling of the face, lips, tongue, or throat) — while less common than with ACE inhibitors — can still occur with ARBs and is a medical emergency.

Losartan vs. Lisinopril: A Quick Comparison

Both losartan and lisinopril block the RAAS system and are highly effective at lowering blood pressure and protecting kidneys and hearts. The main practical differences: lisinopril causes a dry cough in up to 20% of patients (losartan does not), while both can cause angioedema (though the rate is higher with ACE inhibitors). Both carry the same black box pregnancy warning. Lisinopril is generally preferred as a first-line choice in many guidelines because of a longer evidence base, but losartan is an equally valid option, especially when cough is a concern.

Key Drug Interactions

Potassium-sparing diuretics (spironolactone, triamterene), potassium supplements, and other RAAS-blocking drugs (ACE inhibitors, aliskiren) all increase the risk of dangerous hyperkalemia. NSAIDs (ibuprofen, naproxen) can reduce losartan's blood pressure-lowering effect and impair kidney function when combined. Lithium levels can rise when taken with losartan. Fluconazole (an antifungal) can inhibit losartan's conversion to its active metabolite, reducing its effectiveness.

Frequently Asked Questions

Why doesn't losartan cause a cough like lisinopril?

ACE inhibitors like lisinopril block the enzyme that breaks down bradykinin, causing bradykinin to accumulate in the airways and trigger a dry cough in up to 20% of patients. ARBs like losartan act at a different step — they block the angiotensin receptor directly without affecting bradykinin breakdown. No bradykinin accumulation means no cough.

Can losartan protect the kidneys in diabetics?

Yes. Losartan is specifically FDA-approved to slow the progression of diabetic nephropathy in patients with type 2 diabetes and proteinuria. It reduces the pressure within the kidney's filtering units (glomeruli), which in turn slows kidney damage. This benefit is considered to be partially independent of its blood pressure-lowering effect.

What are the signs of high potassium (hyperkalemia) with losartan?

Hyperkalemia can cause muscle weakness, fatigue, irregular heartbeat, numbness, or tingling. In severe cases, it can cause life-threatening cardiac arrhythmias. The risk is higher with kidney disease, potassium supplements, potassium-sparing diuretics, or combined RAAS blockade. Regular blood tests to monitor potassium and kidney function are an important part of safe losartan use.