Onset of Action of Furosemide (Lasix)
Furosemide has an intravenous onset of action within 5 minutes and an oral onset of action within 1 hour, with peak effects occurring within 30-120 minutes after administration. 1
Route-Specific Onset of Action
Intravenous Administration
- Onset: Within 5 minutes 1
- Peak effect: Within 30 minutes 1
- Duration of action: Approximately 2 hours 1
Oral Administration
- Onset: Within 1 hour 2, 1
- Peak effect: 60-120 minutes in most patients 3
- Duration of action: Approximately 2 hours 1
Pharmacokinetic Considerations
The rapid onset of action of furosemide is due to its mechanism of inhibiting sodium and chloride reabsorption in the loop of Henle 1. Several factors can affect the onset and efficacy:
- Bioavailability: Oral furosemide has variable bioavailability (60-64% compared to IV) 1
- Absorption site: Direct administration into the duodenum results in faster absorption than oral administration 4
- Protein binding: Furosemide is 91-99% bound to plasma proteins at therapeutic concentrations 1
- Patient factors:
Clinical Implications
Hemodynamic Effects
- Furosemide can cause transient (1-2 hours) worsening of hemodynamics after administration, including: 2, 5
- Increased heart rate
- Increased mean arterial pressure
- Increased left ventricular filling pressure
- Decreased stroke volume
- Increased systemic vascular resistance
Therapeutic Response
- In congestive heart failure patients, a significant diuretic and natriuretic effect is observed within 60-120 minutes of administration 3
- Response is related to the concentration of the drug in urine rather than in plasma 6
- The diuretic effect correlates positively with furosemide renal clearance and urinary excretion 7
Monitoring Recommendations
When administering furosemide, monitor:
- Response within 6-8 hours of administration 5
- Electrolytes and renal function within 3-5 days of initiation 5
- Blood pressure in both sitting and standing positions to detect orthostatic hypotension 5
- Signs of electrolyte disturbances, particularly hypokalemia, hyponatremia, and hyperuricemia 5
Important Considerations
- For acute heart failure, the initial IV dose should be at least equivalent to the oral home dose 5
- Combining furosemide with vasodilators like nitrates may prevent the transient hemodynamic worsening that can occur with furosemide alone 2, 5
- Avoid furosemide in patients with hypotension or hypovolemia 5
- Tolerance to furosemide can develop with prolonged use, requiring dose adjustments 6
Understanding the rapid onset but relatively short duration of action of furosemide is crucial for appropriate timing of administration and monitoring of therapeutic effects in clinical practice.