What is the recommended oral dose of Lasix (furosemide) when transitioning a patient from intravenous (IV) to oral administration?

IV to Oral Furosemide Transition

When transitioning from IV to oral furosemide, double the IV dose for the oral equivalent, as oral bioavailability is approximately 50% of IV administration. 1

Conversion Ratio

• The standard conversion is 1:2 (IV:PO) - if a patient is receiving 40 mg IV furosemide, transition to 80 mg oral furosemide 1, 2
• Oral furosemide has highly variable bioavailability (averaging 50-60%), which necessitates higher oral doses to achieve equivalent diuretic effect 2
• The poor oral bioavailability is attributed to poor drug solubility, site-specific absorption, and potential presystemic metabolism 2

Dosing Strategy After Conversion

• Administer the oral dose once or twice daily (e.g., at 8 AM and 2 PM if divided dosing is needed) 1
• If a single daily dose is insufficient, the same dose can be given 6-8 hours later, or the dose may be increased by 20-40 mg increments 1
• The individually determined dose should be titrated based on clinical response, with doses carefully increased no sooner than 6-8 hours after the previous dose 1

Clinical Monitoring

• Monitor urine output, symptoms, renal function, and electrolytes regularly during the transition 3
• The diuretic response correlates with urinary furosemide excretion rate and concentration, not plasma levels 2, 4
• Peak diuretic effect from oral furosemide typically occurs within 60-120 minutes 5

Important Caveats

• Patients with severe heart failure and diuretic resistance may require much higher doses - up to 600 mg/day can be safely used in clinically severe edematous states with careful monitoring 1, 6
• In refractory cardiac failure, high-dose furosemide (≥500 mg/day) has been used successfully with mean maintenance doses of 700 mg/day 6
• Consider continuous IV infusion rather than oral conversion if the patient shows signs of diuretic resistance, oliguria despite IV therapy, or hemodynamic instability 3, 7
• For patients not responding adequately to optimized oral furosemide, consider adding a thiazide diuretic or spironolactone for sequential nephron blockade rather than further increasing loop diuretic dose 3, 7

Patient Selection for Oral Conversion

• Only transition to oral therapy when the patient is clinically stable, adequately perfused, and able to absorb oral medications 8
• Avoid oral conversion in critically ill patients or those with gut edema/malabsorption 8
• The combination of high-dose nitrates with lower-dose furosemide may be more effective than aggressive diuretic monotherapy alone in acute heart failure 3