Side Effects of Losartan: Common, Serious & When to Worry

If you’ve just started losartan or are considering it, you’ve probably already wondered whether the fatigue you’re feeling is normal — or whether that slight dizziness warrants a call to your doctor. Most people on this medication experience at least one noticeable side effect within the first few weeks, according to patient reports compiled by the NHS and Mayo Clinic.

Most Common Side Effects: Dizziness, fatigue ·
Serious Signs: Irregular heartbeat, muscle cramps ·
High Potassium Symptoms: Weakness, pins and needles ·
Allergic Reactions: Rash, swelling of face or throat ·
Kidney Risk: Nausea, vomiting

The following table summarizes key drug attributes that most readers check when starting losartan.

What are the major side effects of losartan?

Major side effects fall into two buckets: common but usually harmless, and rare but serious enough to warrant a call to your doctor or, in some cases, emergency care.

Common effects like dizziness and fatigue

Dizziness or lightheadedness is the side effect most frequently reported by people taking losartan, according to patient data from GoodRx and Ubie Health. This happens because losartan lowers blood pressure, and your body needs time to adjust — typically within the first few days to weeks. Fatigue follows closely, with some people describing it as a general heaviness or lack of energy that makes ordinary tasks feel harder than usual. Nasal congestion, back pain, and headache also appear regularly in clinical trial summaries and patient reports. The NHS notes that headaches specifically tend to improve after the first week of consistent dosing.

Serious effects including high potassium

High potassium levels (hyperkalemia) represent one of the more dangerous but uncommon side effects of losartan. According to Ubie Health, elevated potassium can cause muscle weakness, irregular heartbeat, and nausea, though it sometimes produces no symptoms at all — which is why routine blood monitoring matters. The risk is higher in patients who already have kidney disease, take potassium supplements, or use potassium-sparing diuretics, as noted in research from Ubie Health. Muscle cramps that don’t match typical exercise-related cramping, combined with weakness or an irregular heartbeat, should prompt a call to your doctor, the NHS guidance specifies. Severe stomach pain can indicate an inflamed pancreas — a rare but serious reaction documented by the NHS. Yellowing of the eyes or skin points to possible liver involvement and also requires prompt medical attention.

“If you feel weak, have an irregular heartbeat, pins and needles, and muscle cramps, these can be signs of high potassium — contact your GP or seek medical advice,” according to NHS guidance on losartan side effects.

The implication: these serious signs often arrive quietly, without the dramatic fanfare you’d expect from a true emergency. The subtle combination of weakness plus tingling plus irregular heartbeat is exactly the pattern that warrants blood work, not just reassurance.

What are the most common side effects of losartan?

The most frequently occurring side effects are mild and tend to resolve as your body adapts to the medication.

Dizziness and fatigue details

A 2019 study of approximately 2,900 hypertensive patients found that headache and dizziness occurred at rates similar to placebo, suggesting losartan is generally well-tolerated (WithPower Clinical Trials). Ubie Health specifically lists dizziness, lightheadedness, fatigue, and nasal congestion as the primary body reactions to watch for during the adjustment period. Medical News Today confirms fatigue, nasal congestion, and back pain among the common complaints, while also noting chest pain in people with diabetic nephropathy. A dry cough — distinct from the more severe breathing difficulties — shows up as a mild side effect in some patients, according to Medical News Today.

Mild stomach upset

Nausea and diarrhea appear on the list of possible gastrointestinal side effects, per GoodRx and Medical News Today. The Mayo Clinic also notes unusual tiredness or weakness as a more common complaint, along with sore throat, sneezing, and chills — symptoms that can overlap with a common cold and sometimes mask an actual infection versus a drug reaction. Urinary tract infections and itching round out the milder spectrum, according to Medical News Today.

The pattern: the most common losartan side effects are exactly the kind of vague, annoying symptoms that are easy to dismiss — which is precisely why tracking when they started relative to your first dose matters.

What are the long term side effects of losartan?

Long-term use introduces specific concerns centered on kidney function and sustained potassium elevation.

Kidney function changes

Long-term losartan use can cause kidneys not to work as well as they should, the NHS warns. This is not a rare occurrence in absolute terms — it’s a known pharmacological effect of angiotensin receptor blockers that requires monitoring. Doctors check kidney function with regular blood tests during extended losartan therapy, according to NHS guidance. Acute kidney failure, while serious, appears as a documented but uncommon outcome, per Medical News Today. Anemia — a low level of red blood cells — also shows up as a serious long-term effect in some patients, as documented by Medical News Today.

Persistent reactions

Sprue-like enteropathy — a condition causing severe intestinal problems — emerged as a rare but documented long-term reaction to losartan, per GoodRx. This condition, which mimics celiac disease symptoms, has been specifically linked to long-term ARB use. Low blood pressure risks accumulate in some long-term users, particularly those taking other antihypertensives simultaneously, according to GoodRx. Sexual side effects, including erectile dysfunction, can occur with losartan, though Medical News Today notes these are not among the most frequently reported reactions.

The catch: “long-term” doesn’t automatically mean “worse” for most patients — but it does mean the stakes for regular checkups rise considerably.

What should I avoid taking with losartan?

Drug interactions and supplement conflicts deserve as much attention as the side effects themselves — some combinations can amplify risks dangerously.

Other medicines

Potassium supplements, potassium-sparing diuretics (such as spironolactone or amiloride), and ACE inhibitors taken alongside losartan increase the risk of hyperkalemia significantly, based on clinical guidance from Ubie Health and the NHS. Non-steroidal anti-inflammatory drugs (NSAIDs) like ibuprofen or naproxen can reduce losartan’s blood pressure-lowering effect and may worsen kidney function — a particular concern given losartan’s existing kidney-related risks. Lithium-based medications can interact with losartan, requiring careful monitoring by your prescriber.

Herbal supplements

Potassium-containing herbal supplements and salt substitutes that use potassium chloride rather than sodium chloride carry the same hyperkalemia risk as pharmaceutical potassium sources, according to clinical guidance. St. John’s Wort can reduce losartan effectiveness by increasing its metabolism. Always disclose any supplements — prescription or over-the-counter — to your doctor before starting losartan.

“Severe or continuing nausea, vomiting, or stomach pain — especially if they persist beyond a day or two — should be reported to your doctor,” according to Mayo Clinic guidance on losartan side effects.

What this means: the supplement aisle at your pharmacy is not a neutral space when you’re on losartan. Check labels for potassium content, and ask your pharmacist if you’re unsure.

Is losartan a high risk medication?

Compared to many prescription drugs, losartan sits in the moderate-risk category — serious enough to require monitoring, but generally well-tolerated in clinical practice.

Rare but serious risks

Angioedema — swelling of the face, mouth, throat, or neck — represents the most urgent rare risk. The NHS specifies that sudden swelling of lips, mouth, throat, or tongue requires immediate emergency action, including calling 999. This reaction can develop at any point during treatment, not just when starting, as noted by GoodRx. Allergic reactions broadly (rash, hives, itching beyond mild skin irritation) also fall into the serious-but-rare category, per GoodRx. Nitrosamine impurities discovered in some losartan products led to manufacturer recalls, though regulatory agencies determined levels corresponded to published allowable