What is the equivalent oral dose of furosemide (Lasix) when converting from an intravenous (IV) dose?

Furosemide IV to Oral Conversion

When converting from intravenous (IV) to oral furosemide, use a 2:1 ratio - meaning the oral dose should be approximately twice the IV dose to achieve equivalent diuretic effect.

Pharmacokinetic Basis for Conversion

The need for a higher oral dose is based on furosemide's bioavailability characteristics:

• According to the FDA label, the bioavailability of oral furosemide is approximately 60-64% of the IV dose 1
• This reduced bioavailability occurs due to:
  • First-pass metabolism in the liver
  • Incomplete absorption from the gastrointestinal tract
  • Enhanced glucuronidation in certain patient populations 2

Conversion Algorithm

1. Calculate the oral equivalent: Multiply IV dose by 2

   • Example: 40 mg IV furosemide = 80 mg oral furosemide

2. Consider patient-specific factors that may affect conversion:

   • Patients with cirrhosis may require lower doses due to altered metabolism 3
   • Patients with renal impairment may need dose adjustments based on severity
   • Elderly patients should generally start at lower doses 1

Clinical Evidence Supporting Conversion

The 2:1 ratio is supported by multiple lines of evidence:

• The FDA label indicates that oral bioavailability is 60-64% compared to IV administration 1
• Research studies have demonstrated bioavailability of oral furosemide ranging from 51-71% 4, 5
• Some studies suggest even lower bioavailability (around 40%) compared to bumetanide (80%) 6

Dosing Considerations

• Initial oral dosing typically ranges from 20-80 mg as a single dose 1
• Timing: Administer oral furosemide in the morning to avoid nighttime diuresis
• For maintenance therapy, the dose should be adjusted to the minimum effective level
• Maximum daily dose can be up to 600 mg in severe edematous states, though doses exceeding 80 mg/day require careful monitoring 1

Special Populations

• Elderly: Start at the lower end of the dosing range due to potential for reduced renal function and increased sensitivity 1
• Pediatric: Initial oral dose is 2 mg/kg body weight; maximum recommended dose is 6 mg/kg 1
• Renal impairment: Higher doses may be required due to reduced drug delivery to site of action

Common Pitfalls to Avoid

1. Underdosing: Using a 1:1 conversion ratio will likely result in inadequate diuresis

2. Ignoring timing: The peak effect of oral furosemide occurs within 1-2 hours, with duration of 6-8 hours 1

3. Failure to monitor: When converting from IV to oral, monitor:

   • Diuretic response (urine output)
   • Electrolytes (particularly potassium)
   • Renal function
   • Blood pressure

4. Overlooking absorption issues: Food may delay absorption but does not significantly reduce bioavailability 4

By following this 2:1 conversion ratio and considering patient-specific factors, you can effectively transition patients from IV to oral furosemide therapy while maintaining appropriate diuretic effect.