Mechanism of action
Furosemide inhibits the Na-K-2Cl cotransporter (NKCC2) on the luminal membrane of the thick ascending limb of the loop of Henle. By blocking this transporter, furosemide prevents the reabsorption of approximately 25% of filtered sodium, making it the most potent class of diuretics.
The blockade of NKCC2 has several downstream effects:
- Loss of the positive luminal potential: Normally, potassium recycling through ROMK channels creates a positive luminal charge that drives paracellular reabsorption of calcium and magnesium. Furosemide eliminates this gradient, causing hypocalcemia and hypomagnesemia.
- Decreased medullary tonicity: The thick ascending limb is responsible for generating the corticomedullary osmotic gradient. Loop diuretics impair urinary concentration, leading to isosthenuric urine.
- Increased distal sodium delivery: More sodium reaches the collecting duct, where aldosterone-mediated exchange increases potassium and hydrogen ion secretion, causing hypokalemia and metabolic alkalosis.
- Venodilation: Furosemide causes direct venodilation before the diuretic effect begins, providing rapid symptomatic relief in acute pulmonary edema.
Furosemide also stimulates prostaglandin release, contributing to renal vasodilation and increased renal blood flow.
- Furosemide inhibits the Na-K-2Cl cotransporter in the thick ascending limb of the loop of Henle
- Causes hypokalemia, hypomagnesemia, hypocalcemia, metabolic alkalosis, and ototoxicity
- More effective than thiazides when GFR <30 mL/min/1.73 m²
- Ototoxicity risk increased with rapid IV push, high doses, and concurrent aminoglycosides or cisplatin
- NSAIDs reduce efficacy primarily by inhibiting prostaglandin synthesis and decreasing renal blood flow
- Hypokalemia potentiates digoxin toxicity; always monitor potassium in patients on both drugs
- Increases calcium excretion (unlike thiazides, which decrease it); useful for hypercalcemia
- Causes contraction alkalosis and increased distal H+ secretion, leading to metabolic alkalosis
Indications
Furosemide is indicated for conditions requiring rapid or aggressive diuresis:
- Acute decompensated heart failure: First-line agent for acute pulmonary edema and volume overload
- Chronic heart failure: Used when thiazides are insufficient or GFR is reduced
- Acute kidney injury with volume overload: May convert oliguric to non-oliguric AKI, though does not improve mortality
- Nephrotic syndrome: Manages edema when serum albumin is low
- Cirrhosis with ascites: Often combined with spironolactone in a 40:100 mg ratio
- Hypertension: Reserved for patients with renal impairment or resistant hypertension
- Hypercalcemia: Increases calcium excretion after volume repletion with normal saline
- Syndrome of inappropriate ADH secretion (SIADH): Adjunct to fluid restriction
Furosemide is preferred over thiazides when the glomerular filtration rate is below 30 mL/min/1.73 m², as thiazides become ineffective at this threshold.
Pharmacokinetics
Furosemide exhibits variable oral bioavailability and rapid onset of action:
| Parameter | Value |
|---|---|
| Oral bioavailability | 40-70% (variable, food decreases absorption) |
| Onset (IV) | 5 minutes |
| Onset (oral) | 30-60 minutes |
| Peak effect (IV) | 30 minutes |
| Peak effect (oral) | 1-2 hours |
| Duration | 6-8 hours |
| Half-life | 1.5-2 hours (prolonged in renal/hepatic impairment) |
| Protein binding | >95% |
| Metabolism | Minimal hepatic glucuronidation |
| Excretion | 65% renal (unchanged), 35% hepatic/biliary |
Furosemide must be secreted into the tubular lumen via organic anion transporters in the proximal tubule to reach its site of action. This explains why:
- NSAIDs reduce furosemide efficacy primarily by inhibiting cyclooxygenase (COX), which decreases prostaglandin synthesis and reduces renal blood flow, thereby diminishing the diuretic effect; they may also compete for organic anion transporters
- Higher doses are needed in renal impairment to achieve therapeutic tubular concentrations
- Continuous infusion may be more effective than bolus dosing in refractory edema
Dosing follows the product label and is individualized based on response and renal function.
Adverse effects
Furosemide causes predictable electrolyte disturbances and other dose-related toxicities:
Electrolyte and metabolic abnormalities
- Hypokalemia: Most common; increases risk of arrhythmias, especially in patients on digoxin
- Hypomagnesemia: Often overlooked; contributes to refractory hypokalemia
- Hypocalcemia: Results from loss of the positive luminal potential in the thick ascending limb
- Hyponatremia: Free water retention despite sodium loss
- Metabolic alkalosis: Contraction alkalosis plus increased distal H+ secretion
- Hyperuricemia: Reduced uric acid excretion; may precipitate gout
- Hyperglycemia: Impaired insulin secretion and increased insulin resistance
- Hyperlipidemia: Mild increase in LDL and triglycerides
Ototoxicity
Furosemide can cause reversible or irreversible hearing loss and tinnitus, particularly with:
- Rapid IV administration
- High doses
- Concurrent aminoglycosides or cisplatin
- Renal impairment
The mechanism involves disruption of the Na-K-2Cl cotransporter in the stria vascularis of the inner ear.
Other adverse effects
- Volume depletion and hypotension: Excessive diuresis may cause prerenal azotemia
- Pancreatitis: Rare but recognized association
- Interstitial nephritis: Hypersensitivity reaction
- Photosensitivity: Increased risk of sunburn
- Blood dyscrasias: Thrombocytopenia, agranulocytosis (rare)
- Sulfonamide hypersensitivity: Cross-reactivity possible in sulfa-allergic patients
Contraindications and warnings
Absolute contraindications:
- Anuria unresponsive to a trial of furosemide
- Severe electrolyte depletion (uncorrected hypokalemia, hyponatremia)
- Hepatic coma or severe electrolyte disturbances in hepatic cirrhosis
Relative contraindications and warnings:
- Sulfonamide hypersensitivity: Furosemide contains a sulfonamide moiety; use with caution in patients with documented sulfa allergy, though cross-reactivity is uncommon
- Gout: May precipitate acute attacks due to hyperuricemia
- Diabetes mellitus: Hyperglycemia may worsen glycemic control
- Severe hypovolemia: Correct volume status before initiating therapy
- Bladder outflow obstruction: May precipitate acute urinary retention
Special populations:
- Pregnancy (Category C): Not teratogenic but may reduce placental perfusion; use only when benefit outweighs risk
- Breastfeeding: Excreted in breast milk and may suppress lactation
- Elderly: Increased risk of hypotension, electrolyte abnormalities, and falls
- Hepatic cirrhosis: Risk of precipitating hepatic encephalopathy due to electrolyte shifts and volume depletion; use with potassium-sparing diuretics
- Renal impairment: Higher doses may be required; monitor for ototoxicity
Drug interactions
Furosemide has numerous clinically significant drug interactions:
Increased toxicity risk
- Aminoglycosides: Additive ototoxicity and nephrotoxicity (especially gentamicin, tobramycin)
- Cisplatin: Markedly increased risk of irreversible ototoxicity
- Digoxin: Hypokalemia increases digoxin toxicity and risk of arrhythmias
- Lithium: Furosemide reduces lithium clearance, increasing risk of toxicity
- Nephrotoxic agents: Additive renal injury with NSAIDs, amphotericin B, cyclosporine
Reduced diuretic efficacy
- NSAIDs: Inhibit prostaglandin-mediated renal vasodilation, reducing renal blood flow and diminishing the diuretic effect; may also compete for tubular secretion via organic anion transporters
- Probenecid: Competes for tubular secretion, reducing furosemide delivery to its site of action
- Phenytoin: Reduces furosemide absorption and tubular secretion
Electrolyte-related interactions
- ACE inhibitors/ARBs: Additive hypotension; combined therapy increases risk of AKI in volume-depleted states
- Corticosteroids: Additive hypokalemia
- Amphotericin B: Severe hypokalemia and hypomagnesemia
- Laxatives: Additive potassium loss
Cardiovascular interactions
- Antihypertensives: Synergistic blood pressure lowering; may require dose adjustment
- Succinylcholine: Hypokalemia potentiates neuromuscular blockade
- Levothyroxine: High-dose furosemide may displace thyroid hormone from binding proteins
Clinical pearl: Always monitor potassium levels when furosemide is combined with digoxin, as hypokalemia dramatically increases the risk of digoxin-induced arrhythmias.
Clinical pearls
Diuretic resistance
Patients may develop tolerance to furosemide through:
- Distal tubule hypertrophy: Chronic loop diuretic use causes compensatory sodium reabsorption in the distal convoluted tubule
- Decreased renal perfusion: Heart failure, cirrhosis, or volume depletion reduces drug delivery
- Post-diuretic sodium retention: Avid sodium retention during the "off" period between doses
Strategies to overcome resistance include increasing dose, switching to continuous infusion, or adding a thiazide diuretic (sequential nephron blockade).
Loop vs. thiazide diuretics
Loop diuretics are preferred when:
- GFR <30 mL/min/1.73 m²
- Acute pulmonary edema requiring rapid diuresis
- Hypercalcemia (loops increase calcium excretion; thiazides decrease it)
- Severe volume overload
Thiazides are preferred for:
- Hypertension as monotherapy
- Osteoporosis prevention (thiazides reduce calcium excretion)
- Hypercalciuria and calcium nephrolithiasis
Furosemide in acute decompensated heart failure
The DOSE trial showed that bolus versus continuous infusion and high-dose versus low-dose strategies produced similar outcomes, though high-dose therapy improved dyspnea more quickly at the cost of transient worsening of renal function.
Monitoring
Routine monitoring should include:
- Serum electrolytes (sodium, potassium, magnesium, calcium)
- Renal function (BUN, creatinine)
- Volume status (weight, blood pressure, orthostatics)
- Uric acid in patients with gout
- Glucose in diabetic patients
Exam tip
In a patient with hypokalemic metabolic alkalosis, think loop or thiazide diuretics. If there is also hypocalcemia and hypomagnesemia, it is a loop diuretic (thiazides cause hypercalcemia).
OH DANG for loop diuretic side effects: Ototoxicity, Hypokalemia, Dehydration, Alkalosis (metabolic), Nephritis (interstitial), Gout (hyperuricemia)
Practice questions
1. A 68-year-old man with chronic heart failure presents with acute dyspnea, bilateral crackles, and peripheral edema. He is started on IV furosemide. Three days later, his edema has improved but he develops muscle cramps and an ECG shows flattened T waves and prominent U waves. Which electrolyte abnormality is most likely responsible?
2. A 72-year-old woman with nephrotic syndrome is treated with high-dose IV furosemide for severe edema. She is also receiving gentamicin for a urinary tract infection. Two days later, she reports ringing in her ears and difficulty hearing conversations. Which mechanism best explains her symptoms?
3. A 55-year-old man with cirrhosis and ascites is started on furosemide and spironolactone. Laboratory studies show Na+ 128 mEq/L, K+ 3.0 mEq/L, Cl- 88 mEq/L, HCO3- 32 mEq/L, and arterial pH 7.50. Which acid-base disorder is present?
Frequently asked questions
How does furosemide work?
Furosemide works by inhibiting the Na-K-2Cl cotransporter in the thick ascending limb of the loop of Henle, blocking sodium and chloride reabsorption. This prevents approximately 25% of filtered sodium from being reabsorbed, making it the most potent class of diuretics. The blockade also disrupts the positive luminal potential needed for paracellular calcium and magnesium reabsorption, leading to hypocalcemia and hypomagnesemia. Additionally, increased sodium delivery to the distal tubule promotes potassium and hydrogen ion secretion, causing hypokalemia and metabolic alkalosis.
What are the side effects of furosemide?
The most common side effects are electrolyte abnormalities: hypokalemia, hypomagnesemia, hypocalcemia, and hyponatremia. Furosemide also causes metabolic alkalosis, hyperuricemia (which may trigger gout), hyperglycemia, and hyperlipidemia. Ototoxicity is a serious adverse effect that can cause reversible or irreversible hearing loss, especially with high doses, rapid IV administration, or concurrent use of aminoglycosides or cisplatin. Other side effects include volume depletion, hypotension, photosensitivity, and rarely pancreatitis or interstitial nephritis.
Is furosemide a controlled substance?
No, furosemide is not a controlled substance. It is a prescription loop diuretic used to treat fluid overload and edema. While not controlled, it does require a prescription and should be used under medical supervision due to its potential to cause significant electrolyte disturbances, volume depletion, and other adverse effects. Furosemide has been misused by athletes for rapid weight loss and to mask performance-enhancing drug use, which has led to its prohibition by many sports organizations.
Furosemide vs hydrochlorothiazide — what is the difference?
Furosemide is a loop diuretic that acts on the thick ascending limb and is more potent than hydrochlorothiazide, a thiazide diuretic that acts on the distal convoluted tubule. Furosemide blocks approximately 25% of sodium reabsorption versus 5-10% for thiazides. Furosemide works even when GFR is below 30 mL/min, while thiazides become ineffective at this threshold. Furosemide increases calcium excretion and causes hypocalcemia, whereas hydrochlorothiazide decreases calcium excretion and may cause hypercalcemia. Thiazides are preferred for hypertension and osteoporosis prevention, while furosemide is used for acute heart failure and severe volume overload.
Why does furosemide cause metabolic alkalosis?
Furosemide causes metabolic alkalosis through two mechanisms. First, it causes contraction alkalosis: as volume is lost through diuresis, bicarbonate concentration in the blood increases. Second, by blocking sodium reabsorption in the loop of Henle, more sodium reaches the collecting duct where aldosterone drives sodium-potassium and sodium-hydrogen exchange. The increased hydrogen ion secretion into the urine raises blood pH, creating metabolic alkalosis. This is typically accompanied by hypokalemia, which further stimulates renal hydrogen ion secretion.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: C03CA01
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.