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Drug Reference

furosemide

Loop diuretic· Brands: LasixIVPO

Mechanism of action
Furosemide inhibits the sodium-potassium-2 chloride cotransporter (NKCC2) in the thick ascending limb of the loop of Henle. This reduces reabsorption of sodium, chloride and water, increasing urine output. Furosemide also increases renal prostaglandin synthesis, which can dilate renal vessels and enhance diuresis. It may also affect potassium and calcium ion transport, causing increased excretion of these ions.
Pharmacokinetics
Oral furosemide has a bioavailability of approximately 50% with wide inter-individual variation. Peak plasma concentrations occur 1 to 2 hours after oral administration, while the onset of diuresis is within 1 hour for oral and 5 minutes for intravenous use. About 95% of the drug binds to plasma albumin. It is metabolised primarily by glucuronidation in the liver, and the metabolites are excreted in the urine, along with unchanged furosemide, via tubular secretion and glomerular filtration. The elimination half-life is approximately 1 to 3 hours in healthy adults, but is prolonged in renal impairment, neonates, and the elderly, leading to cumulative effects.
Indications & dosing

Edema due to heart failure, hepatic cirrhosis, or renal disease

Initial dose 20-40 mg; adjust according to response. Monitor electrolytes and renal function.

  • adult · PO: 20–80 mg once daily · Initial dose 20-40 mg; adjust according to response. Monitor electrolytes and renal function.

Acute pulmonary edema

Give slow IV injection. If response inadequate, repeat after 20 min.

  • adult · IV: 20–40 mg once · Give slow IV injection. If response inadequate, repeat after 20 min.

Hypertension

May be used alone or in combination; not first-line for uncomplicated hypertension.

  • adult · PO: 20–40 mg twice daily · May be used alone or in combination; not first-line for uncomplicated hypertension.

Edema in pediatric patients

Dose per kg body weight. Lower doses for infants. Monitor fluid balance and electrolytes.

  • pediatric · PO: 1–2 mg/kg once daily · Dose per kg body weight. Lower doses for infants. Monitor fluid balance and electrolytes.
Contraindications & precautions
  • Anuria · Do not use in patients with no urine output.
  • Hypersensitivity to furosemide or sulphonamides · Cross-sensitivity may occur.
  • Severe hypokalaemia · Correct potassium before initiating therapy.
  • Hepatic coma or precoma · Risk of precipitating encephalopathy.
  • Severe hyponatraemia · Correct sodium before initiating therapy.
  • Uncorrected hypovolaemia · Restore volume status before administration.
  • Monitor serum electrolytes, renal function and blood pressure regularly, especially during initial titration.
  • Use caution in patients with hepatic cirrhosis, as rapid fluid shifts may precipitate encephalopathy.
  • Caution in diabetes mellitus, as furosemide may impair glucose tolerance.
  • Caution in gout, as increased serum uric acid may precipitate attacks.
  • Avoid use in uncorrected hypovolaemia; correct volume status before administration.
  • Caution with concomitant use of aminoglycosides or other ototoxic drugs.
Adverse effects
  • Hypokalaemiaserious
  • Hyponatraemiaserious
  • Hypotensionserious
  • Dehydrationserious
  • Ototoxicityserious
  • Acute kidney injuryserious
  • Hyperuricaemia
  • Hyperglycaemia
  • Metabolic alkalosis
  • Gastrointestinal upset
Interactions
  • ibuprofenmajorAvoid chronic co-administration if possible. If required, monitor renal function, blood pressure, and fluid balance at baseline and periodically thereafter. Use the lowest effective ibuprofen dose for the shortest duration, and consider alternative analgesia such as paracetamol when appropriate. Adjust the furosemide dose as needed to maintain adequate diuresis.
  • digoxinmajorMonitor serum potassium, magnesium, renal function, and digoxin levels before and during co-administration. Correct hypokalaemia with potassium supplementation or potassium-sparing diuretics as appropriate. Adjust digoxin dose if renal function deteriorates. Advise patients to report symptoms of toxicity promptly. Consider ECG monitoring in high-risk patients.
  • gentamicinmajorAvoid concurrent use if possible. If unavoidable, ensure adequate hydration, monitor renal function (serum creatinine and urea) and hearing before and during therapy. Adjust gentamicin doses based on renal function and consider alternative diuretics or antibacterial agents when appropriate.
  • lithiummajorAdvise the patient to maintain adequate fluid and sodium intake. Avoid thiazides if possible, as they reduce lithium clearance even more than loop diuretics; if an alternative diuretic is needed, use one with minimal effect on lithium handling and monitor levels closely.
Renal & hepatic adjustment
  • eGFR 0–30: In severe renal impairment, higher doses may be required to achieve diuresis. Use with caution; monitor for ototoxicity and volume depletion.
  • Hepatic impairment (severe): Use with caution; risk of electrolyte imbalance and hepatic encephalopathy. Monitor electrolytes and mental status.
Monitoring, pregnancy & toxicity
  • Serum electrolytes (potassium, sodium, magnesium) · Baseline and regularly, especially with long-term use or digoxin co-administration
  • Renal function (serum creatinine, eGFR) · Baseline and periodically; more frequently if impaired
  • Fluid balance (daily weight, input/output) · Daily in hospital; weekly in outpatients
  • Blood pressure · Regularly, especially in hypertension or with other antihypertensives
  • Blood glucose · In patients with diabetes or at risk
  • Uric acid · In patients with gout
  • Pregnancy: Not recommended in pregnancy unless clearly needed (e.g., pulmonary oedema). Use only after specialist consultation; may reduce placental perfusion and cause fetal electrolyte disturbance.
  • Toxicity: Hypovolaemia and hypotension · Replace fluids (oral or IV) with isotonic saline; correct volume status; monitor blood pressure and urine output.
  • Toxicity: Electrolyte disturbances (hypokalaemia, hyponatraemia, hypochloraemia) · Monitor electrolytes; replace potassium and sodium as needed; monitor ECG for arrhythmias.
  • Toxicity: Metabolic alkalosis · Correct with fluids and electrolytes; treat underlying cause; monitor acid-base balance.
  • Toxicity: Ototoxicity (high doses, rapid IV) · Discontinue furosemide; consider other diuretics; monitor hearing; symptoms may be reversible.
Clinical notes
Furosemide is indicated for oedema associated with heart failure, cirrhosis, and renal disease, and for the treatment of hypertension when other diuretics are inadequate. It is also used intravenously for acute pulmonary oedema. As a high-ceiling loop diuretic, it carries risks of hypokalaemia, hyponatraemia, metabolic alkalosis, dehydration, and ototoxicity, particularly with rapid intravenous administration or in patients with renal impairment. It may elevate blood glucose and urate levels, and can precipitate gout. Electrolyte concentrations, renal function, and blood pressure should be monitored regularly. Furosemide interacts with aminoglycosides and other ototoxic drugs, increasing the risk of hearing loss. Dosage varies substantially by indication and patient factors; always check local prescribing guidelines and adjust according to response.

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BNF; local formulary