What is the equivalent dose of losartan (Angiotensin II Receptor Blocker) when switching a patient from irbesartan (Angiotensin II Receptor Blocker) 75mg?

Irbesartan 75mg to Losartan Conversion

When switching from irbesartan 75mg to losartan, start with losartan 50mg once daily and titrate to 100mg once daily as the target dose.

Dose Equivalence Rationale

The conversion is based on comparative efficacy data showing that irbesartan demonstrates superior blood pressure reduction compared to losartan at their respective starting doses. Irbesartan 150mg is approximately equivalent to losartan 100mg in antihypertensive efficacy 1. Therefore, irbesartan 75mg (half the standard starting dose) corresponds to approximately losartan 50mg, with titration to 100mg as the therapeutic target 1.

Evidence from Comparative Trials

• A randomized controlled trial directly comparing irbesartan and losartan demonstrated that irbesartan 150mg produced significantly greater reductions in diastolic blood pressure (-10.2 mm Hg) compared to losartan 50mg (-7.9 mm Hg) at 8 weeks 1
• When doses were doubled, irbesartan 300mg continued to show superior efficacy over losartan 100mg, with response rates of 78% versus 64% respectively 1
• A multicenter trial comparing multiple ARBs found irbesartan 150mg reduced sitting diastolic blood pressure by 9.9 mm Hg compared to 8.2 mm Hg with losartan 50mg 2

Recommended Conversion Protocol

Initial conversion: Start losartan 50mg once daily when discontinuing irbesartan 75mg 3, 4

Titration schedule:

• Assess blood pressure and tolerability after 2 weeks 3
• If blood pressure control is inadequate and the medication is well-tolerated, increase to losartan 100mg once daily 3, 4
• The target dose for optimal cardiovascular outcomes is 100mg daily for hypertension, though 150mg daily may be considered for heart failure 5, 3

Critical Monitoring Parameters

Within 1-2 weeks of conversion, check:

• Serum creatinine and estimated glomerular filtration rate 3
• Serum potassium (risk of hyperkalemia, especially with chronic kidney disease or diabetes) 3
• Blood pressure in both sitting and standing positions (assess for orthostatic hypotension) 3

Ongoing monitoring:

• Recheck electrolytes and renal function at least annually during maintenance therapy 3
• Monitor blood pressure regularly to ensure adequate control 3

Important Clinical Considerations

Unique Properties of Each ARB

• Losartan has uricosuric effects that irbesartan does not possess, decreasing serum uric acid by approximately 47 μmol/L 6. This may be beneficial in patients with hyperuricemia or gout 6
• Irbesartan may have modest effects on uric acid and oxidative stress markers, though less pronounced than losartan 7

Dosing Frequency Options

• Losartan can be administered once or twice daily, though once-daily dosing is preferred for adherence 3
• The total daily dose should not exceed 100mg for hypertension (150mg for heart failure) 3
• If twice-daily dosing is needed for better 24-hour coverage, split the total dose (e.g., 50mg twice daily for 100mg total) 3

Common Pitfalls to Avoid

Never combine losartan with ACE inhibitors or direct renin inhibitors - this combination increases risks of hyperkalemia, syncope, and acute kidney injury without providing additional cardiovascular benefit 5, 3

Avoid underdosing - less than 25% of patients in clinical practice are titrated to target doses, which compromises therapeutic efficacy 3. The HEAAL trial demonstrated that losartan 150mg daily was superior to 50mg daily with a 10% relative risk reduction in death or heart failure hospitalization 5, 3

Do not discontinue prematurely for mild hyperkalemia - implement potassium-lowering strategies (dietary modification, diuretic adjustment, potassium binders) before stopping the medication 3

Monitor for hypotension during titration - particularly in elderly patients, those with heart failure, or volume-depleted patients 3. Start at lower doses and titrate more gradually in these populations 3

Special Population Considerations

• Hepatic impairment: Start losartan at 25mg daily due to 5-fold increase in plasma concentrations 3
• Heart failure with reduced ejection fraction: Target dose is 100-150mg daily based on the HEAAL trial 5, 3
• Diabetic nephropathy: Target dose is 100mg once daily for patients with albuminuria 3
• Pregnancy: Discontinue immediately - losartan causes serious fetal toxicity 3