Losartan Side Effects
Losartan is generally very well tolerated with a side effect profile similar to placebo, though serious adverse effects including hypotension, hyperkalemia, angioedema, and fetal harm require careful monitoring.
Common Side Effects
The most frequently reported side effects in patients with hypertension include 1:
- Upper respiratory infections ("colds") - most common
- Dizziness - the only side effect reported more frequently than placebo (2.4% vs 1.3%)
- Stuffy nose
- Back pain
- Headache
In patients with type 2 diabetes and diabetic kidney disease, additional common effects include 1:
- Diarrhea
- Fatigue/tiredness
- Low blood sugar (hypoglycemia)
- Chest pain
Notably, dry cough occurs in only 3.1% of losartan patients versus 8.8% with ACE inhibitors, making it an excellent alternative for patients who cannot tolerate ACE inhibitor-related cough 2.
Serious Adverse Effects Requiring Immediate Action
Hypotension (Low Blood Pressure)
- May cause faintness or dizziness
- Particularly dangerous in hospitalized patients: Recent evidence shows hypotension occurred in 30.4% of hospitalized COVID-19 patients on losartan versus 15.3% on usual care 3
- Action: Lie down if feeling faint; call physician immediately 1
Hyperkalemia (High Potassium)
- Increased risk especially with 4, 5:
- Chronic kidney disease
- Concurrent potassium supplements
- Potassium-sparing diuretics (spironolactone, triamterene)
- Salt substitutes containing potassium
- Avoid combining losartan with other drugs affecting the renin-angiotensin system (ACE inhibitors, aliskiren) 4, 5
Angioedema (Allergic Reaction)
- Swelling of face, lips, throat, or tongue
- Action: Stop losartan immediately and seek emergency medical help 1
- Rare but documented in postmarketing surveillance 6
- If prior ACE inhibitor-related angioedema, wait 6 weeks before starting losartan 5
Pregnancy-Related Harm (Category D)
Most critical warning: Losartan causes fetal injury and death when used in second and third trimesters 1:
- Reduces fetal renal function
- Causes oligohydramnios (low amniotic fluid)
- Results in fetal lung hypoplasia, skeletal deformations, skull hypoplasia
- Can cause neonatal anuria, hypotension, renal failure, and death
- Discontinue immediately when pregnancy is detected 1
Worsening Renal Function
- Monitor for swelling in feet, ankles, hands, or unexplained weight gain 1
- Risk of acute renal failure in patients with severe bilateral renal artery stenosis 4, 5
Drug Interactions to Monitor
Tell your physician if taking 1:
- NSAIDs (including COX-2 inhibitors) - may reduce antihypertensive effect
- Lithium - losartan may increase lithium levels
- Diuretics - may potentiate hypotension
- Aliskiren - contraindicated in diabetic patients
Clinical Context and Safety Profile
Multiple large trials involving approximately 3,800 patients demonstrate that losartan's overall adverse event rate (2.3% discontinuation) is lower than placebo (3.7%) 2. The drug shows similar tolerability across age groups, including elderly patients, with no significant metabolic abnormalities or laboratory changes 7, 8.
First-dose hypotension is uncommon due to losartan's slower onset of action compared to ACE inhibitors 6, 2. Rebound hypertension upon discontinuation has not been observed 2.
Special Populations
- Breastfeeding: Unknown if excreted in human milk; choose either losartan or breastfeeding, not both 1
- Hepatic impairment: Use cautiously; losartan is metabolized by liver 9
- Renal impairment: Monitor closely for worsening function 1
- Black patients with left ventricular hypertrophy: May be less effective than atenolol for reducing cardiovascular events 1
When to Contact Your Physician
Call immediately if experiencing:
- Dizziness or fainting
- Swelling of face, lips, or throat
- Unexplained weight gain or leg swelling
- Pregnancy or planning pregnancy
- Persistent bothersome side effects