Furosemide
Furosemide is a potent loop diuretic used to treat fluid overload (oedema) in heart failure, kidney disease and liver cirrhosis, and as a second-line option for hypertension. Marketed as Lasix and as authorised generics in oral and intravenous forms.
- Chemical formula
- C12H11ClN2O5S
- CAS number
- 54-31-9
- ATC code
- C03CA01
- Molecular weight
- 330.7 g/mol
- Drug class
- Loop diuretic
- Also known as
- Frusemide, Lasix
What is it?
Furosemide is the most widely used loop diuretic, approved in the 1960s and on the WHO Essential Medicines List. It was developed and originally marketed by Hoechst (now Sanofi) under the brand name Lasix, with authorised generics widely available since the 1980s. Furosemide is one of the most prescribed cardiovascular medications worldwide, particularly for the management of fluid overload in heart failure, chronic kidney disease and cirrhosis, and as a second-line antihypertensive when first-line options are insufficient.
Mechanism of action
Furosemide acts on the thick ascending limb of the loop of Henle in the kidney, where it inhibits the Na+/K+/2Cl- co-transporter (NKCC2). This blocks reabsorption of sodium, chloride, potassium and water, producing a strong diuretic effect. By rapidly reducing total body water and sodium, furosemide decreases circulating blood volume and lowers cardiac filling pressures, which relieves pulmonary and peripheral congestion. The molecule also has minor effects on calcium and magnesium handling and on renal blood flow.
Pharmacokinetics
After oral administration, furosemide is absorbed within 30 to 60 minutes, with peak plasma concentrations in 1 to 2 hours. Oral bioavailability is variable (50-70%), particularly reduced in heart failure patients due to gut wall oedema. The molecule is highly protein bound and is eliminated mainly through renal excretion of the unchanged drug. The terminal half-life is approximately 1.5 to 2 hours in healthy adults but is prolonged in renal impairment. Onset after intravenous dosing is within 5 minutes.
Indications
Furosemide is approved for the treatment of fluid overload due to heart failure, chronic kidney disease and liver cirrhosis (with or without ascites), as well as for acute pulmonary oedema. It is also approved for hypertension, although thiazide diuretics are usually preferred as first-line in this indication. Intravenous furosemide is used in hospital for rapid relief of acute fluid overload. The molecule is not approved for weight loss or as a routine treatment of mild dependent oedema without an underlying medical cause.
Safety profile
Common adverse effects include electrolyte imbalances (low potassium, magnesium, sodium, calcium), volume depletion, dizziness on standing, and increased serum uric acid (with potential gout flares). Higher doses or rapid intravenous infusion can cause ototoxicity, particularly in patients with renal impairment. Allergic reactions, including cross-reactivity with sulfonamide antibiotics, are uncommon but documented. According to the prescribing information, regular monitoring of electrolytes, kidney function and blood pressure is required during treatment.
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Frequently asked questions
How quickly does furosemide work? ▾
Onset after an oral dose is typically 30 to 60 minutes, with peak diuretic effect within 1 to 2 hours and overall duration of effect of approximately 6 hours. Intravenous furosemide acts within 5 minutes and is preferred in hospital for acute fluid overload. The strong, rapid diuresis can produce a noticeable increase in urination within an hour of oral dosing — a practical consideration when planning daytime activities, according to clinical guidelines.
What dose of furosemide is typical? ▾
The starting dose depends on indication and severity. For chronic oedema, typical adult oral starting doses range from 20 to 40mg once or twice daily, with adjustment up to 600mg per day in some refractory cases. For mild hypertension, lower doses are used. Intravenous doses of 20 to 80mg are common in acute fluid overload. According to the prescribing information, dose decisions must be individualised based on clinical response, kidney function and electrolyte balance.
Why does furosemide cause low potassium? ▾
Furosemide blocks the NKCC2 transporter in the loop of Henle, increasing sodium delivery to the distal nephron, where sodium reabsorption is exchanged for potassium and hydrogen ion secretion. The result is increased urinary potassium loss. According to the prescribing information, regular potassium monitoring is required, and potassium supplements or potassium-sparing co-therapy may be needed. Severe hypokalaemia can cause muscle weakness, cramps and cardiac arrhythmias.
Can furosemide be used during pregnancy? ▾
Furosemide is generally not recommended in pregnancy unless the benefits clearly outweigh the risks, because it crosses the placenta and may reduce maternal blood volume. According to clinical guidelines, alternative treatments are preferred for pregnancy-related oedema and hypertension. In specific conditions where furosemide is necessary (severe heart failure, pulmonary oedema), it is used under specialist supervision with careful monitoring.
What interactions are clinically important? ▾
Furosemide has clinically relevant interactions with several drug classes. Combination with aminoglycoside antibiotics increases ototoxicity risk; with NSAIDs reduces diuretic efficacy and increases kidney injury risk; with lithium increases lithium levels and toxicity. Combinations with other antihypertensives can cause excessive blood pressure drops, particularly with first doses. According to the prescribing information, all current medications should be reviewed before starting furosemide and during dose changes.
The information on this website is provided for reference and educational purposes only. It does not replace consultation with a qualified healthcare professional.