What intravenous furosemide dose corresponds to an 80 mg oral Lasix dose in an adult patient?

IV Furosemide Equivalent for 80 mg Oral Dose

For an adult patient taking 80 mg oral furosemide, the equivalent intravenous dose is 40 mg IV, reflecting the approximately 50% oral bioavailability of furosemide.

Bioavailability and Conversion Rationale

• Oral furosemide has an average bioavailability of approximately 50% (range 10-90%), meaning only about half of an oral dose reaches the systemic circulation and the renal tubular site of action. 1, 2, 3

• The standard conversion is 2:1 (oral:IV), so 80 mg PO converts to approximately 40 mg IV. 1

• Despite delivering only half the amount of drug to the active site, oral and IV routes produce similar overall natriuretic responses because oral administration maintains drug concentrations at the maximally efficient portion of the dose-response curve for a longer duration. 2

Clinical Context for Route Selection

• IV administration is preferred in acute situations requiring rapid diuresis (onset within minutes vs. approximately 1 hour for oral), such as acute decompensated heart failure or pulmonary edema. 4, 1

• Oral administration is preferred for chronic management due to convenience, good bioavailability in stable patients, and avoidance of acute reductions in glomerular filtration rate associated with IV boluses. 4, 1

• In heart failure exacerbations or cirrhosis with gut wall edema, oral absorption may be further impaired, potentially requiring the full oral dose equivalent (80 mg PO → 80 mg IV) rather than the standard 2:1 conversion. 1

Practical Dosing Algorithm

When converting 80 mg oral furosemide to IV:

• For stable patients with preserved gut absorption: Start with 40 mg IV (standard 2:1 conversion). 1

• For patients with acute decompensated heart failure or prior diuretic exposure: Consider starting with 40-80 mg IV, as guidelines recommend using at least the oral equivalent dose when switching routes in acute settings. 4, 1

• For patients already on chronic oral furosemide ≥80 mg/day: The initial IV dose should be at least equivalent to their oral dose (80 mg IV) or even 2-2.5× their home dose for acute decompensation. 4

Monitoring After Conversion

• Place a bladder catheter in acute settings to monitor urine output hourly, targeting >0.5 mL/kg/h as evidence of adequate diuretic response. 4

• Check electrolytes (particularly potassium and sodium) and renal function within 6-24 hours after starting IV furosemide, then every 3-7 days during active titration. 4

• Target daily weight loss of 0.5 kg (without peripheral edema) to 1.0 kg (with peripheral edema) until dry weight is achieved. 4

Common Pitfalls to Avoid

• Do not under-dose IV furosemide by rigidly applying the 2:1 conversion in patients with acute volume overload who have been on chronic oral therapy; these patients often require the full oral equivalent or higher IV doses. 4, 1

• Recognize that gut wall edema in heart failure reduces oral bioavailability, making the IV route more reliable and potentially requiring higher IV doses than the standard conversion suggests. 1

• Avoid giving IV furosemide to hypotensive patients (systolic BP <90-100 mmHg) without circulatory support, as it worsens tissue perfusion rather than improving hemodynamics. 4

• Do not exceed 160-200 mg as a single IV bolus, and consider continuous infusion (5-10 mg/hour after a loading dose) when daily requirements exceed 160 mg rather than giving repeated high-dose boluses. 4