What is the equivalent oral (PO) dose of Furosemide (Lasix) when converting from intravenous (IV) administration?

From the Guidelines

When converting Lasix (furosemide) from intravenous (IV) to oral (PO) administration, the recommended conversion ratio is 2:1, meaning that the oral dose should be approximately twice the IV dose, as supported by the 2016 ESC guidelines for the diagnosis and treatment of acute and chronic heart failure 1. This conversion is necessary because oral furosemide has a bioavailability of approximately 50-60%, while IV administration has 100% bioavailability.

Key Considerations

• The initial IV dose should be at least equal to the pre-existing oral dose used at home, and the oral dose should be adjusted accordingly to achieve the desired clinical effect.
• Patients with new-onset AHF or those with chronic HF without a history of renal failure and previous use of diuretics may respond to IV boluses of 20 – 40 mg, whereas those with previous use of diuretics usually require higher doses.
• Factors such as gastrointestinal function, food intake, and concurrent medications may influence oral absorption, and patients should be assessed for appropriate diuretic response, electrolyte balance, and renal function after conversion.

Clinical Implications

• The timing of administration may also need adjustment, as IV furosemide acts within minutes while oral furosemide typically takes 30-60 minutes to begin working.
• Close monitoring of the patient's response is crucial, as individual variations in absorption can affect the clinical outcome, and adjustments to the oral dose may be necessary to achieve optimal diuresis.
• The conversion approach ensures continued effective diuresis while transitioning to a more convenient oral administration route, which can improve patient outcomes and quality of life.

From the FDA Drug Label

Adults: Parenteral therapy with Furosemide Injection should be used only in patients unable to take oral medication or in emergency situations and should be replaced with oral therapy as soon as practical. The usual initial dose of furosemide is 20 to 40 mg given as a single dose, injected intramuscularly or intravenously The dose may be raised by 20 mg and given not sooner than 2 hours after the previous dose until the desired diuretic effect has been obtained This individually determined single dose should then be given once or twice daily.

The equivalent oral (PO) dose of Furosemide (Lasix) when converting from intravenous (IV) administration is 20 to 40 mg given orally, as the initial dose. However, the exact equivalent oral dose is not explicitly stated for IV doses, and the conversion should be individualized according to patient response to gain maximal therapeutic response and to determine the minimal dose needed to maintain that response 2.

From the Research

Equivalent Oral Dose of Furosemide

When converting from intravenous (IV) administration to oral (PO) administration, the equivalent dose of Furosemide (Lasix) can be determined based on the drug's bioavailability.

• The bioavailability of oral furosemide is approximately 40% to 70% 3.
• This means that the oral dose of furosemide may need to be higher than the IV dose to achieve the same therapeutic effect.
• For example, if a patient is receiving 40 mg of IV furosemide, the equivalent oral dose could be around 60 mg to 100 mg, assuming a bioavailability of 40% to 60% 3.

Clinical Studies

Several clinical studies have investigated the use of furosemide in patients with heart failure and volume overload.

• One study found that the median daily loop diuretic dose was 240 mg (IQR: 80 to 800 mg) oral furosemide or equivalent 4.
• Another study found that high-dose loop diuretics were associated with hypokalemia and hyponatremia, but not with reduced survival after multivariate and propensity adjustment 5.
• A study comparing furosemide and bumetanide found that bumetanide has an estimated bioavailability of 80%, compared to approximately 40% for furosemide 3.

Dosing Considerations

When converting from IV to oral furosemide, it is essential to consider the patient's individual response to the drug and adjust the dose accordingly.

• The recommended initial dose of oral furosemide is 20-40 mg/d, with a maximum dose of 160 mg/d 6.
• The dose may need to be adjusted based on the patient's response to treatment, as well as their renal function and other comorbidities.
• Close monitoring of the patient's condition, including their weight, blood pressure, and electrolyte levels, is crucial to ensure safe and effective treatment 4, 5.