Equivalence of IV and PO Furosemide (Lasix)
Yes, 40mg of intravenous (IV) furosemide is generally considered equivalent to 80mg of oral (PO) furosemide, reflecting the approximately 50% oral bioavailability of furosemide. 1, 2
Pharmacokinetic Basis for the Equivalence
- Furosemide has poor and variable oral bioavailability, with approximately 30-40% of an oral dose being absorbed and reaching systemic circulation compared to IV administration 2
- On a milligram basis, IV furosemide is approximately three times as potent as oral furosemide due to this bioavailability difference 1
- The peak effect of IV furosemide occurs within 30 minutes, while oral furosemide peaks at approximately 75 minutes 1
Clinical Implications of Route Selection
- IV administration provides more rapid onset of action and is preferred in acute situations requiring immediate diuresis 3
- Oral administration is generally preferred for chronic management due to convenience and the good oral bioavailability in stable patients with cirrhosis 3
- The acute reductions in glomerular filtration rate associated with IV furosemide administration may favor oral administration in certain clinical scenarios 3
Dosing Considerations
- When converting from IV to oral therapy, doubling the dose (e.g., from 40mg IV to 80mg PO) is the standard approach to account for bioavailability differences 1, 2
- For patients with heart failure, the typical starting doses are 20-40mg IV or 40-80mg PO 3
- In patients with cirrhosis and ascites, the usual starting dose is 40mg PO furosemide (often combined with 100mg spironolactone) 3
Monitoring Response
- Response to furosemide is related to the concentration of the drug in urine rather than in plasma 4
- The diuretic effect is relatively short-lived for both IV and oral formulations, typically returning to baseline within 3.5-4 hours 1
- Patients with different underlying conditions may have variable responses to equivalent doses 5
Potential Pitfalls
- Bioavailability can vary significantly between individuals, so the 1:2 ratio (IV:PO) is an approximation 4
- In certain disease states (e.g., heart failure exacerbation, cirrhosis), oral absorption may be further impaired, potentially requiring higher oral doses 3
- IV furosemide can cause transient worsening of hemodynamics in heart failure patients, including increased heart rate, mean arterial pressure, left ventricular filling pressure, and decreased stroke volume 3
- Excessive or rapid diuresis with either formulation can lead to electrolyte abnormalities and dehydration 3