What is the equivalent dose of furosemide (Lasix) to torasemide (generic) when given intravenously (IV) versus orally (PO)?

Equivalent Dosing of Furosemide and Torasemide for IV and PO Administration

The equivalent dose ratio of furosemide to torasemide is 4:1 for oral administration (40 mg furosemide PO = 10 mg torasemide PO) and 1:1 for intravenous administration (20 mg furosemide IV = 20 mg torasemide IV). 1

Intravenous (IV) Dosing Equivalence

When administering these loop diuretics intravenously:

• 20 mg IV furosemide = 20 mg IV torasemide
• 40 mg IV furosemide = 40 mg IV torasemide

The 2016 ESC guidelines specifically mention that 10-20 mg IV torasemide may be considered as an alternative to 20-40 mg IV furosemide, suggesting a 1:1 ratio for IV administration 1.

Oral (PO) Dosing Equivalence

For oral administration, the dosing equivalence is different:

• 40 mg PO furosemide = 10 mg PO torasemide
• 80 mg PO furosemide = 20 mg PO torasemide
• 160 mg PO furosemide = 40 mg PO torasemide

This 4:1 ratio for oral dosing is supported by multiple guidelines and research evidence 1, 2.

Pharmacokinetic Differences Explaining Dosing Ratios

The difference in dosing ratios between IV and PO administration is primarily due to:

1. Bioavailability differences:

   • Torasemide has approximately 80-90% oral bioavailability 3
   • Furosemide has only 26-65% oral bioavailability 3
   • This explains why less torasemide is needed orally to achieve the same effect

2. Duration of action:

   • Torasemide: 12-16 hours 1
   • Furosemide: 6-8 hours 1

3. Metabolism:

   • Torasemide undergoes extensive hepatic metabolism with only 20% excreted unchanged in urine 3
   • Furosemide has minimal metabolism with 80-90% excreted unchanged in urine 3

Clinical Considerations

When switching between these medications or routes of administration:

• When converting from IV to PO furosemide, use the same dose (1:1 ratio)
• When converting from IV to PO torasemide, use a 2:1 ratio (20 mg IV = 10 mg PO)
• When converting between oral furosemide and torasemide, use the 4:1 ratio

Dosing in Special Populations

• Renal impairment: Both drugs require dose adjustments, but torasemide's pharmacokinetics are less affected by renal dysfunction 2
• Hepatic impairment: Torasemide's clearance may be reduced in severe liver disease 3

Common Pitfalls to Avoid

1. Assuming the same ratio for both IV and PO administration - The equivalence ratio differs between routes
2. Underdosing when switching from oral furosemide to oral torasemide - Remember the 4:1 ratio
3. Overlooking the longer duration of action of torasemide - May affect dosing frequency
4. Failing to monitor electrolytes - Both medications can cause electrolyte disturbances

Recent evidence from the TRANSFORM-Mechanism trial suggests that clinicians typically use a 2:1 dose equivalence conversion between these drugs in practice, though a 4:1 ratio resulted in more similar natriuresis 4. This highlights the importance of following established guidelines for initial dosing while monitoring clinical response.