How should I switch a patient on irbesartan 150 mg to losartan, including initial dosing, need for washout, monitoring, and titration?

Switching from Irbesartan 150 mg to Losartan

Direct Conversion Without Washout

Switch directly from irbesartan 150 mg to losartan 100 mg once daily without any washout period, as both are angiotensin-II receptor blockers with similar mechanisms and no pharmacokinetic interaction risk. 1

The conversion is straightforward because:

• Irbesartan 150 mg is approximately equivalent to losartan 100 mg in antihypertensive efficacy, based on comparative trials showing similar blood pressure reductions at these doses. 2, 3
• No washout period is required when switching between ARBs, as there is no risk of overlapping toxicity or drug interaction. 4
• Losartan 100 mg once daily represents the maximum FDA-approved dose for hypertension and the target dose recommended by ACC/AHA guidelines. 1, 5

Initial Dosing Strategy

Start losartan at 100 mg once daily (not 50 mg) to maintain equivalent blood pressure control when converting from irbesartan 150 mg. 6

The rationale for starting at the full dose:

• Clinical trials demonstrate that irbesartan 150 mg produces greater blood pressure reduction than losartan 50 mg, with comparable efficacy to losartan 100 mg. 2, 3
• Starting at a lower dose (50 mg) would result in temporary loss of blood pressure control and require unnecessary titration. 1
• Patients already tolerating an ARB at therapeutic doses can safely transition to the equivalent dose of another ARB. 6

Monitoring Protocol

Check blood pressure within 2–4 weeks after switching, and measure serum creatinine and potassium within 1–2 weeks. 1, 6

Key monitoring parameters:

• Blood pressure reassessment at 2–4 weeks is essential because individual response may vary despite population-level dose equivalence. 1, 6
• Serum creatinine/eGFR and potassium within 1–2 weeks after the switch, particularly in patients with diabetes, chronic kidney disease, or baseline renal impairment. 1
• Target blood pressure goal is <130/80 mmHg for most adults to reduce cardiovascular risk. 1

Titration and Escalation

If blood pressure remains uncontrolled (≥140/90 mmHg) after 4–8 weeks on losartan 100 mg:

• Add hydrochlorothiazide 12.5–25 mg once daily rather than exceeding the maximum losartan dose, as combination therapy provides additive blood pressure-lowering effects. 1, 5
• Do not increase losartan beyond 100 mg daily for hypertension, as this is the FDA-approved maximum dose; higher doses (150 mg) are reserved for heart failure with reduced ejection fraction. 5
• Consider adding a dihydropyridine calcium channel blocker (e.g., amlodipine 5–10 mg) if triple therapy is needed. 1

Critical Safety Considerations

Never combine losartan with an ACE inhibitor or direct renin inhibitor (aliskiren), as dual RAAS blockade increases the risk of hyperkalemia, syncope, and acute kidney injury by 2–3-fold without added cardiovascular benefit. 1, 6

Additional safety points:

• Losartan is absolutely contraindicated in pregnancy due to serious fetal toxicity (renal dysfunction, oligohydramnios, skull hypoplasia, fetal death); discontinue immediately if pregnancy is detected. 1
• Monitor for symptomatic hypotension, especially in elderly or volume-depleted patients, though this is uncommon when switching between ARBs at equivalent doses. 1
• Losartan has a favorable drug interaction profile with no clinically significant interactions with warfarin, digoxin, or hydrochlorothiazide. 4

Practical Administration Details

• Losartan can be taken at any time of day without regard to food, which may improve adherence. 1, 4
• Once-daily dosing is preferred for the 100 mg dose, though the total daily dose may be split into 50 mg twice daily if more consistent 24-hour coverage is desired. 1, 6
• Peak plasma concentrations occur 1–2 hours post-dose, with the active metabolite E-3174 having a half-life of 6–9 hours. 4

Common Pitfalls to Avoid

• Do not start at losartan 50 mg when converting from irbesartan 150 mg, as this represents underdosing and will result in inadequate blood pressure control. 2, 3
• Do not delay the switch or attempt a gradual cross-titration; direct substitution is safe and appropriate. 4
• Do not forget to recheck electrolytes and renal function within 1–2 weeks, as this is a critical safety measure often overlooked in clinical practice. 1
• Do not exceed losartan 100 mg daily for hypertension alone, as this increases hyperkalemia risk without additional benefit; instead, add a second agent from a different class. 5