What are the side effects of losartan potassium?

October 9, 2025 •

4 min read

In clinical trials, the overall incidence of adverse events with losartan was similar to that of a placebo, indicating that the medication is generally well-tolerated. However, it is crucial for patients to understand what are the side effects of losartan potassium to ensure safe and effective use of the medication.

Quick Summary

Losartan potassium can cause side effects ranging from mild issues like dizziness and fatigue to serious conditions such as high potassium, low blood pressure, or kidney problems. Understanding and monitoring these potential effects is key for patient safety, requiring communication with a healthcare provider about any concerns.

Key Points

Common side effects are typically mild: Losartan often causes mild and temporary side effects like dizziness, fatigue, and cold-like symptoms, which usually subside as your body adjusts to the medication.
Hyperkalemia is a serious risk: Losartan can cause dangerously high levels of potassium in the blood, especially in patients with kidney problems, diabetes, or those using potassium supplements.
Angioedema is a rare but life-threatening allergic reaction: Swelling of the face, lips, tongue, or throat can occur and requires immediate emergency medical attention.
Losartan is unsafe during pregnancy: The drug carries a boxed warning and can cause severe harm or death to an unborn baby, particularly in the second and third trimesters.
Regular monitoring is crucial: Healthcare providers will need to regularly monitor your kidney function and electrolyte levels, especially your potassium, while you are taking losartan.
Drug interactions are a concern: Patients should be cautious with over-the-counter NSAIDs and avoid potassium supplements without a doctor's guidance due to interaction risks.
Communication with your doctor is vital: Never stop taking losartan or change your dosage without consulting your healthcare provider, even if you experience side effects.

Understanding Losartan Potassium

Losartan potassium, often known by its brand name Cozaar®, is a prescription medication belonging to a class of drugs called angiotensin II receptor blockers (ARBs). It is primarily used to treat high blood pressure (hypertension), but also has applications in managing diabetic nephropathy in type 2 diabetes and reducing the risk of stroke in patients with left ventricular hypertrophy. By blocking the action of angiotensin II, a hormone that constricts blood vessels, losartan helps to relax and widen them, which lowers blood pressure and makes it easier for the heart to pump blood.

While considered a highly effective and well-tolerated treatment, patients should be aware of the range of side effects that can occur. These can be categorized into common, less common, and serious effects, and vary depending on the patient's underlying health conditions and concurrent medications.

Common Side Effects of Losartan Potassium

Many patients experience minimal to no side effects, and those that do occur are often mild and transient, particularly during the initial weeks of treatment.

Commonly reported side effects include:

Dizziness and lightheadedness: These are among the most frequently reported side effects and may occur, especially when standing up too quickly. It is often a sign of low blood pressure.
Upper respiratory infections: Patients may experience cold-like symptoms such as a stuffy or runny nose, sore throat, or sinus infections.
Back pain: Muscle or back pain is a potential side effect.
Fatigue or weakness: A general feeling of unusual tiredness can occur.
Diarrhea: Stomach upset and diarrhea are sometimes reported.
Headache: Headaches are also a common complaint, particularly early in treatment.

Managing common side effects: For mild issues like dizziness, moving slowly when changing positions can help. Resting and staying hydrated can relieve headaches and fatigue. If any common side effects persist or become bothersome, it is best to discuss them with a healthcare provider.

Serious Side Effects and Contraindications

While rare, some side effects are serious and require immediate medical attention. It is crucial to be vigilant for these signs and communicate with a healthcare professional promptly if they occur.

Hyperkalemia (High Potassium Levels)

Losartan can cause an increase in serum potassium levels, known as hyperkalemia. This can be dangerous and, in severe cases, life-threatening. The risk is higher for patients with kidney problems, diabetes, heart failure, or those taking other medications that increase potassium.

Symptoms of high potassium may include:

Muscle weakness or unusual tiredness
Numbness or tingling sensations
Slow or irregular heartbeat
Chest pain
Nausea

Patients taking losartan should not use potassium supplements or salt substitutes containing potassium without first consulting their doctor.

Angioedema (Serious Allergic Reaction)

Angioedema is a rare but severe allergic reaction involving the swelling of the face, lips, tongue, or throat. This can lead to difficulty breathing and requires immediate emergency medical care. Patients who have previously experienced angioedema with another medication, such as an ACE inhibitor, may be at increased risk.

Kidney Function Impairment

Changes in kidney function, including potential acute kidney failure, can occur, especially in patients with pre-existing kidney conditions, severe heart failure, or those who are volume-depleted. Healthcare providers will monitor kidney function with regular blood tests. Signs of kidney problems include reduced urination, swelling in the feet or ankles, and unusual weakness or tiredness.

Fetal Toxicity (Pregnancy Warning)

Losartan carries a boxed warning due to the significant risk of fetal harm or death when taken during the second and third trimesters of pregnancy. It should be discontinued as soon as pregnancy is detected. Women who are pregnant or planning to become pregnant should discuss alternative treatment options with their doctor.

Hypotension (Low Blood Pressure)

Symptomatic low blood pressure, characterized by dizziness and lightheadedness, can occur, particularly when a patient first starts treatment or has low fluid or salt levels.

Drug-Drug Interactions

NSAIDs: Concurrent use of losartan with nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen or naproxen may increase the risk of kidney function deterioration.
Dual Blockade of RAS: Using losartan alongside ACE inhibitors or aliskiren is generally avoided, particularly in patients with diabetes or renal impairment, as it significantly increases the risk of hypotension, hyperkalemia, and kidney issues.
Lithium: Losartan can increase lithium levels, potentially leading to toxicity.

Losartan vs. ACE Inhibitors: A Side Effect Comparison

Losartan is an alternative to ACE inhibitors (like lisinopril) for patients who cannot tolerate the persistent dry cough often associated with ACE inhibitors. The table below highlights key differences in side effect profiles.


Side Effect	Losartan (ARB)	Lisinopril (ACE Inhibitor)
Persistent Dry Cough	Much less likely	More common and often bothersome
Angioedema	Rare risk	Higher, though still relatively low, risk
Hyperkalemia	Possible risk; monitor potassium levels	Possible risk; monitor potassium levels
Renal Impairment	Possible risk; monitor kidney function	Possible risk; monitor kidney function
Fetal Toxicity	Boxed warning against use in pregnancy	Use also contraindicated in pregnancy

Conclusion: Navigating Losartan Safely

While losartan potassium offers significant benefits for managing hypertension and associated conditions, understanding its potential side effects is essential. The majority of patients tolerate the medication well, but it is important to be aware of both common, manageable side effects like dizziness and fatigue, and serious, albeit rare, risks such as angioedema and hyperkalemia. Consistent communication with your healthcare provider is key. They will monitor your kidney function and electrolyte levels, especially if you have pre-existing health conditions or are taking other medications. If you experience any concerning symptoms, particularly swelling, chest pain, or irregular heartbeats, seek medical help immediately. Never adjust your dosage or stop taking losartan without consulting your doctor, as this can have serious health consequences. For comprehensive and up-to-date prescribing information, refer to official sources like the National Institutes of Health (NIH) National Library of Medicine or your healthcare provider's instructions.

Disclaimer: This article provides general information about losartan potassium and is not a substitute for professional medical advice. Always consult with a qualified healthcare provider for any health concerns or before making decisions related to your treatment.

Frequently Asked Questions

The most common side effects of losartan include dizziness, lightheadedness, upper respiratory tract infections (like colds), stuffy nose, back pain, fatigue, and headache.

Yes, losartan can increase potassium levels in the blood, a condition called hyperkalemia. This can be serious, and a doctor will monitor your levels, especially if you have pre-existing kidney issues or diabetes.

No, losartan is an angiotensin II receptor blocker (ARB) and is much less likely to cause the persistent dry cough that is a common side effect of ACE inhibitors like lisinopril.

Angioedema is a rare but serious allergic reaction that can be caused by losartan. It involves swelling of the face, lips, tongue, or throat and requires immediate emergency medical treatment.

No, losartan is not safe during pregnancy, particularly in the second and third trimesters, due to the risk of fetal harm or death. It should be discontinued immediately if pregnancy is detected.

Dizziness is a common side effect, especially when you first start taking the medication. To help prevent it, stand up slowly after sitting or lying down. If you feel faint, lie down and rest until you feel better. If the dizziness persists or worsens, contact your doctor.

You should avoid potassium supplements, potassium-containing salt substitutes, and certain nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen or naproxen unless your doctor approves. Always inform your doctor of all other medications you are taking.

Signs of a serious allergic reaction include difficulty breathing, swelling of the face, tongue, or throat, and hives. Seek emergency medical attention immediately if you experience these symptoms.