Lasix PO to IV Conversion
When converting from oral to intravenous furosemide, use a 1:1 dose equivalence (same milligram dose), as oral furosemide has approximately 50% bioavailability but IV administration achieves higher peak concentrations and more predictable diuresis. 1
Bioavailability and Conversion Rationale
- Oral furosemide has approximately 40-50% bioavailability, meaning only half of an oral dose reaches systemic circulation 2
- Despite lower bioavailability, the standard conversion is 1:1 (same dose IV as PO) because IV administration delivers drug directly to the kidney tubules more rapidly and predictably 1
- The FDA label recommends starting with 20-40 mg IV for edema, which corresponds to typical oral starting doses 1
Practical Conversion Guidelines
For patients already on oral furosemide:
- Convert to the same milligram dose IV when switching routes 3
- Example: A patient on furosemide 40 mg PO daily should receive 40 mg IV 1
- For patients on chronic oral diuretics requiring acute treatment, the IV dose should be at least equivalent to their oral dose 3
Administration considerations:
- IV doses should be given slowly over 1-2 minutes to avoid ototoxicity 1
- For high-dose therapy (>80 mg), consider continuous infusion at a rate not greater than 4 mg/min 1
- Oral administration is preferred in cirrhotic patients due to good bioavailability and avoidance of acute GFR reduction associated with IV administration 4, 3
Important Clinical Caveats
When IV route is preferred:
- Acute situations requiring rapid diuresis (pulmonary edema, acute heart failure) 3
- Patients unable to take oral medication 1
- Emergency situations 1
- Severe volume overload requiring predictable, rapid response 3
Monitoring after conversion:
- Check blood pressure every 15-30 minutes in the first 2 hours 3
- Monitor urine output, targeting 0.5-1.0 mL/kg/h 3
- Assess electrolytes (sodium, potassium) and renal function within 6-24 hours 3, 5
- Watch for signs of excessive diuresis: hypotension, acute kidney injury, electrolyte derangements 3
Common pitfall to avoid:
- Do not assume IV furosemide requires dose reduction due to higher bioavailability—the 1:1 conversion accounts for pharmacokinetic differences and is the standard of care 1, 3
- In cirrhosis specifically, oral route is preferred when possible as IV furosemide can cause acute reduction in renal perfusion 4, 5