Losartan Side Effects: What to Expect and When to Call Your Doctor

Losartan is generally well tolerated by most patients. In clinical trials, the rate of side effects was similar to placebo in many studies. But like all medications, losartan can cause side effects — and some of them are serious enough to require medical attention.

This guide explains the most common and serious side effects of losartan, what symptoms to watch for, and when to call your doctor. This is educational information — always talk to your healthcare provider with specific questions about your situation.

Losartan's Boxed Warning: Fetal Toxicity

Losartan carries the FDA's most serious warning — a boxed warning — for fetal toxicity. When taken during the second or third trimester of pregnancy, losartan can cause injury or death to the developing fetus. This includes:

• Reduced kidney function in the fetus (oligohydramnios)
• Fetal lung underdevelopment
• Skull abnormalities and skeletal deformations
• Death of the newborn

If you become pregnant while taking losartan, stop it immediately and call your doctor. Do not wait. Women of childbearing age taking losartan should use reliable contraception.

Common Side Effects (Reported in ≥2% of Patients)

In clinical trials, the most common side effects of losartan occurring in 2% or more of patients (and more frequently than placebo) were:

• Dizziness — especially when standing up quickly (orthostatic hypotension). Most common in the first few weeks of therapy or after a dose increase. Usually resolves on its own.
• Upper respiratory infection — stuffy or runny nose, sore throat. Unlike ACE inhibitors, losartan does not typically cause chronic dry cough.
• Nasal congestion — reported in some patients, especially early in treatment.
• Back pain — reported in clinical trials, though not clearly caused by the drug in most cases.
• Fatigue — more common in the diabetic nephropathy trial (RENAAL) where patients had more complex medical conditions.
• Anemia — observed in diabetic nephropathy patients on losartan in the RENAAL study.

Serious Side Effects: Call Your Doctor Right Away

These side effects are less common but require prompt medical attention:

• Hyperkalemia (high potassium): Symptoms include nausea, weakness, tingling, chest pain, irregular heartbeat, and loss of movement. This is especially a risk if you take potassium supplements, potassium-sparing diuretics (like spironolactone), or have chronic kidney disease.
• Hypotension (low blood pressure): Severe dizziness, fainting, or lightheadedness. More likely if you're dehydrated, on a diuretic, or have had GI illness (vomiting or diarrhea). Seek medical attention if you faint.
• Kidney problems: Swelling in legs or ankles, urinating much less than usual, or shortness of breath. Losartan can affect kidney function, especially if you have pre-existing kidney disease or are dehydrated. Your doctor monitors your creatinine and potassium with regular blood tests.
• Angioedema: Rare but serious. Swelling of the face, lips, throat, or tongue — can obstruct the airway. This is a medical emergency. Call 911 immediately. Losartan is generally safer than ACE inhibitors for patients with a history of ACE inhibitor-induced angioedema, but it has rarely been reported with ARBs as well.
• Rhabdomyolysis (rare): Unexplained muscle pain, weakness, or tenderness, especially with fever and dark urine. Rare with losartan but possible. Seek care promptly.

What About Dry Cough?

Persistent dry cough is a well-known side effect of ACE inhibitors (like lisinopril), occurring in up to 20% of patients. One of the key advantages of ARBs like losartan is that they do not inhibit ACE, so they do not cause this cough. Losartan was specifically developed as an alternative for patients who couldn't tolerate ACE inhibitor-induced cough.

If you're experiencing a persistent dry cough while taking losartan, it's worth discussing with your doctor — it may be caused by something other than the medication.

Monitoring During Losartan Treatment

Your doctor will typically monitor the following during losartan therapy:

• Blood pressure (at home and office visits)
• Serum potassium levels
• Serum creatinine / kidney function (BMP or CMP)
• Urine albumin (for diabetic nephropathy monitoring)

Learn about what medications interact with losartan: Losartan Drug Interactions: What to Avoid and What to Tell Your Doctor

For a full medication overview: What Is Losartan? Uses, Dosage, and What You Need to Know in 2026.