Furosemide

Overview

Furosemide (Frusemide) is a potent loop diuretic widely used in dogs and cats for the management of fluid overload conditions, particularly those associated with cardiac disease such as congestive heart failure.

It has a rapid onset of action — about 5 minutes after intravenous injection and 30 minutes after intramuscular injection, and 30 to 60 minutes orally in dogs — making it highly effective in emergency situations. The duration of effect is relatively short — roughly 2 to 6 hours depending on the source and route — requiring repeated dosing or transition to oral therapy for ongoing management.

Mechanism of Action (MOA): Furosemide acts by inhibiting the Na+/K+/2Cl− cotransporter in the thick ascending limb of the loop of Henle. This results in increased excretion of sodium, chloride, potassium, and water in the urine. It also enhances the excretion of calcium, magnesium and hydrogen, increases renal blood flow and glomerular filtration rate, and may transiently cause venodilation following intravenous administration, contributing to its rapid clinical effects in acute pulmonary oedema.

Furosemide has no effect on carbonic anhydrase and does not antagonise aldosterone. Furosemide is often called a high-ceiling diuretic because it is more effective than other diuretics; increased intrarenal prostaglandin production also contributes by raising renal blood flow.

Indications

Furosemide is primarily used in dogs and cats for the management of conditions associated with fluid overload and abnormal fluid accumulation. Its potent diuretic effect makes it a cornerstone drug in both emergency and chronic care settings.

• Congestive heart failure (CHF): First-line therapy for the management of pulmonary edema and fluid retention in both acute, life-threatening episodes and chronic heart failure. It is typically used as part of a multimodal protocol rather than as monotherapy for long-term control.
• Hypercalcaemia: Used to promote calciuresis after adequate rehydration, typically lowering serum calcium by 0.5 to 1.5 mmol/L in dogs and cats.
• Acute renal failure / oliguria (selected cases): May be used to stimulate urine production after correction of dehydration, but it increases urine output without improving glomerular filtration rate or clinical outcome, so a rise in urine production should not be read as recovery of renal function.
• Fluid overload states: Utilized to manage edema or effusions associated with cardiac disease, particularly when rapid diuresis is required.
• Hyperkalaemia: Used as adjunctive therapy to increase renal potassium excretion while the underlying cause is addressed.
• Systemic hypertension: Occasionally used as adjunctive therapy for systemic hypertension in dogs with oedema not responding adequately to standard combination treatment.
• Prevention of sterile haemorrhagic cystitis: Given alongside cyclophosphamide chemotherapy in dogs to reduce the incidence of sterile haemorrhagic cystitis.

Dosage (Reference)

Dog

In dogs, furosemide dosing depends on the severity of the condition and clinical response. In emergency situations, repeated parenteral dosing is often required, followed by transition to oral maintenance therapy once the patient stabilizes.

Cat

In cats, furosemide is used cautiously, typically at the lower end of the dosing range, with close monitoring due to increased sensitivity to adverse effects such as dehydration and electrolyte imbalance.

Warnings & Precautions

Furosemide is a potent diuretic with significant effects on fluid balance, electrolytes, and renal function. Careful patient selection, dose adjustment, and ongoing monitoring are essential to avoid complications, especially in dogs and cats with underlying systemic disease.

• Dehydration and hypovolaemia: Rapid or excessive diuresis can cause dehydration; pre-existing dehydration, serious electrolyte depletion and known hypersensitivity are contraindications to use.
• Electrolyte imbalances: Common disturbances include hypokalaemia, hyponatraemia, hypochloraemia, hypocalcaemia, and hypomagnesaemia; regular monitoring is essential during therapy.
• Renal function: May cause or worsen prerenal azotaemia due to reduced renal perfusion; monitor urea, creatinine, and urine output closely.
• Hepatic disease: Use with caution in patients with impaired hepatic function, as fluid and electrolyte shifts may precipitate hepatic coma; furosemide is contraindicated once hepatic coma is present.
• Diabetes mellitus: Use cautiously in patients with diabetes mellitus; furosemide can cause hyperglycaemia, although less than the thiazide diuretics.
• Cardiovascular compromise: A marked reduction in cardiac output may occur in patients with severe pulmonary disease, low-output heart failure, hypertrophic cardiomyopathy, pericardial or myocardial disorders, cardiac tamponade and severe hypertension.
• Pericardial effusion: Contraindicated where cardiac tamponade is confirmed, as diuresis may worsen haemodynamic instability before the effusion is relieved; it can be used after effusion drainage to help manage right-sided heart failure signs if needed.
• Anuria: Contraindicated in patients with anuria.
• Monitoring requirements: Monitor electrolytes, acid-base status, urea, creatinine, glucose, hydration and serial body weight with PCV/TP; also track sleeping or resting respiratory rate, blood pressure, clinical signs of congestion, and hearing where treatment is prolonged, particularly in cats. Thoracic radiographs are indicated where the underlying disease warrants them.
• Pregnancy and lactation: Labelling conflicts between markets — UK product labels state furosemide may be used during pregnancy and lactation, while US labels state it is either not recommended during the second trimester or contraindicated in pregnant animals. Fetal abnormalities, unexplained maternal deaths, abortion and fetal hydronephrosis have all been reported in animal reproduction studies. Weigh risks against benefits in a pregnant patient. Furosemide appears in milk and the significance to nursing offspring is unknown.
• Sulfonamide sensitivity: Furosemide is structurally related to the sulfonamides, and patients hypersensitive to sulfonamides may also react to furosemide, although this has not been documented in veterinary species.
• Early signs of electrolyte imbalance: Watch for increased thirst, lethargy, drowsiness or restlessness, fatigue, reduced urine output, gastrointestinal upset and a fast heart rate; take corrective measures as soon as these appear.
• When to discontinue: Discontinue furosemide in patients with progressive renal disease if azotaemia and oliguria increase during therapy.
• Non-cardiogenic pulmonary oedema: Evidence for efficacy in non-cardiogenic pulmonary oedema is lacking, so furosemide should not be assumed to help every cause of respiratory distress.

Drug Interactions

Most clinically important interactions with furosemide in dogs and cats are related to additive nephrotoxicity, electrolyte disturbances, or altered cardiovascular and neuromuscular effects. Careful monitoring and dose adjustment are required when combining with the following drugs.

• Aminoglycosides (e.g., gentamicin, amikacin): Increased risk of nephrotoxicity and ototoxicity, particularly with concurrent use; avoid or monitor closely.
• Digoxin: Furosemide-induced hypokalaemia may increase the risk of digoxin toxicity; monitor potassium levels carefully.
• Other diuretics (e.g., thiazides, acetazolamide): Concurrent acetazolamide or thiazide diuretics increase the risk of hypokalaemia; monitor potassium closely.
• Corticosteroids and theophylline: Increased risk of hypokalaemia when used concurrently; electrolyte monitoring is recommended.
• NSAIDs: May reduce the diuretic efficacy of furosemide and increase the risk of nephrotoxicity, especially in patients with compromised renal perfusion.
• Neuromuscular blocking agents: Furosemide alters the response to neuromuscular blocking agents: reported effects on non-depolarizing agents such as tubocurarine and atracurium differ between references, with both reduced muscle relaxation and prolonged blockade described, while the effects of depolarizing agents such as suxamethonium are increased.
• ACE inhibitors (e.g., benazepril, enalapril): The standard partner drugs in heart failure, but the combination increases the risk of hypotension and azotaemia, especially in volume- or sodium-depleted patients. Introduce furosemide conservatively and monitor blood pressure, renal values and electrolytes.
• Amphotericin B: Concurrent use raises the risk of nephrotoxicity, ototoxicity and hypokalaemia; monitor renal values, hearing and potassium closely.
• Cisplatin: Combined use increases the risk of nephrotoxicity and ototoxicity, including complete deafness, even at otherwise typical doses of either drug.
• Cephalosporins, oxytetracycline and polymyxin B: Each carries an increased risk of renal injury when given with furosemide; monitor renal values during concurrent use.
• Other reported interactions: Further reported or theoretical interactions: desmopressin increases the risk of hyponatraemia, insulin requirements may change, ciclosporin increases the risk of hyperuricaemia, probenecid’s uricosuric effect is reduced, and excretion of salicylates such as aspirin may fall.

Side Effects & Overdose

Side Effects

Adverse effects of furosemide in dogs and cats are primarily related to its potent diuretic action and resulting fluid and electrolyte losses. Fluid and electrolyte disturbances are less likely when normal food and water intake is maintained.

• Electrolyte imbalances: Commonly includes hypokalaemia, hyponatraemia, hypochloraemia, hypocalcaemia, and hypomagnesaemia.
• Dehydration: May occur readily during furosemide treatment.
• Polyuria and polydipsia: Increased urine output with compensatory water intake is expected during therapy.
• Prerenal azotaemia: May develop if moderate to severe dehydration occurs.
• Reduced cardiac output: A marked reduction in cardiac output may occur in patients with severe pulmonary disease, low-output heart failure, hypertrophic cardiomyopathy, pericardial or myocardial disorders, cardiac tamponade or severe hypertension.
• Ototoxicity: Reported particularly in cats receiving high-dose intravenous therapy, and in dogs at intravenous doses above roughly 20 to 22 mg/kg, with profound hearing loss at 50 to 100 mg/kg. Temporary deafness may occur after rapid intravenous administration of high doses.
• Gastrointestinal disturbances: May include nausea, vomiting, or diarrhoea.
• Hematologic effects: Leucopenia and anaemia have been reported.
• Weakness and restlessness: Both have been reported during treatment.
• Injection-site reactions: Subcutaneous injection can cause ulcerative skin lesions in dogs; formulation pH may be relevant, so use the subcutaneous route with caution and observe the injection site.
• Acid-base disturbance: Metabolic alkalosis may occur; monitor acid-base status alongside electrolytes.
• Sodium loss exceeds potassium loss: In dogs, potassium is lost much less than sodium, so hyponatraemia is generally a greater concern than hypokalaemia and hypochloraemia is common — worth remembering when interpreting an electrolyte panel on a treated patient.
• Hyperglycaemia: Furosemide can raise blood glucose, though less so than thiazide diuretics; relevant when treating a diabetic patient.

Overdose

Overdose of furosemide can result in severe disturbances in fluid balance, electrolytes, and cardiovascular function. Clinical severity depends on the dose and route of administration.

• Severe dehydration and hypovolaemia: Severe water and electrolyte imbalance may progress to cardiovascular collapse.
• Marked electrolyte disturbances: Especially hypokalaemia and hyponatraemia, which may result in weakness.
• Renal impairment: Moderate to severe dehydration may cause prerenal azotaemia.
• Cardiovascular compromise: Cardiovascular collapse may occur in severe acute overdose.
• Ototoxicity: A concern particularly in cats receiving high-dose intravenous therapy; temporary deafness may occur after rapid intravenous administration of high doses.
• Management: Empty the gut after a recent oral overdose, but do not give cathartics — they worsen the fluid and electrolyte losses. Otherwise supportive care: fluid therapy, aggressive correction of electrolyte and water imbalances, and close monitoring of respiratory, neurological and cardiovascular status.
• Reference toxic doses: Reported LD50 values in dogs are greater than 1000 mg/kg orally and about 300 mg/kg intravenously. Chronic administration of 10 mg/kg for 6 months in dogs caused mild dehydration with calcification and scarring of the renal parenchyma. Any overdose still requires prompt assessment for fluid, electrolyte, neurological and cardiovascular complications.
• Neurological signs: Acute overdose may cause neurological signs including lethargy, coma and seizures.

Key Notes

Practical clinical points that help optimize the safe and effective use of furosemide in dogs and cats in everyday veterinary practice:

• Rapid emergency effect: Intravenous administration has a rapid onset and is a first-choice diuretic for acute cardiogenic pulmonary oedema.
• Titrate to effect: Adjust acute dosing according to respiratory rate and respiratory effort rather than using a fixed schedule alone.
• Transition strategy: Stabilized patients should be switched from injectable to oral therapy as soon as possible for long-term management.
• Multimodal CHF therapy: In dogs with chronic heart failure due to mild valvular regurgitation or dilated cardiomyopathy, furosemide should form part of multimodal therapy with pimobendan and an ACE inhibitor rather than be used as diuretic monotherapy.
• Refractory cases: Lack of response at higher doses suggests the need for combination diuretic therapy or switching to an alternative loop diuretic.
• Hydration-first approach: Correct pre-existing dehydration before giving furosemide; dehydration is a contraindication to its use.
• Administration flexibility: In patients with ascites, subcutaneous administration may give better bioavailability and clinical effect than oral dosing.
• Clinical goal: The aim is not maximal diuresis, but achieving adequate control of clinical signs using the lowest effective dose.
• Pharmacokinetics: Oral bioavailability is about 77% in dogs and about 50% in healthy cats, with a reported feline oral half-life of 1.2 hours. The drug is more than 90% protein bound. After intravenous dosing in healthy cats, reported values are a volume of distribution of 227 mL/kg, clearance of 149 mL/kg/h and an elimination half-life of 2.25 hours.
• Onset and peak effect: In healthy dogs urine output peaks about 1 hour after IV or subcutaneous dosing and 2 hours after oral dosing, returning to baseline at roughly 2, 4 and 6 hours respectively. Subcutaneous absorption is comparable to other injectable routes.
• Tolerance with continued use: The diuretic effect wanes with continued use: repeated administration activates the renin-angiotensin-aldosterone system and attenuates the response, with increased RAAS activity documented by day 5 in dogs given 2 mg/kg every 12 hours. Sustained aldosterone elevation has persistent, deleterious effects on vasculature and cardiac remodelling.
• Relative potency: Furosemide is considered approximately one-tenth to one-twentieth as potent as torsemide in dogs and cats. This is a potency comparison, not a validated switching conversion.
• Home monitoring: Ask the owner to record the sleeping or resting respiratory rate at home to help monitor for early signs of recurring cardiac disease.
• Thyroid test interference: Furosemide increased the free thyroxine fraction and inhibited T4 binding to canine serum in vitro; interpret canine thyroid results cautiously during treatment.
• Storage: Protect from light and store at room temperature. Discard opened bottles of oral solution after 90 days, and discard tablets that have discoloured. A precipitate may form if the injection is refrigerated but redissolves on warming without loss of potency. Do not mix with acidic solutions; at 10 mg/mL furosemide may be mixed with 5% dextrose, 0.9% saline or lactated Ringer’s solution for intravenous use, and those mixtures are stable for 8 hours.
• Compatibility: Before mixing or co-infusing furosemide with another drug, check a specialised compatibility reference — compatibility varies with pH, concentration, temperature and diluent.
• Practical dispensing in cats: Many cats are dosed in 6.25 mg increments, which is half of a 12.5 mg tablet — a useful anchor when converting a calculated dose into something the owner can actually give.
• Water access: Ensure continuous access to fresh, clean water. Never restrict water to reduce the frequency of urination on furosemide.
• Signs needing urgent attention: Tell the owner to seek veterinary assessment immediately for weakness, hearing loss, collapse, head tilt, lack of urination or a racing heartbeat.
• Giving with food: Furosemide may be given with or without food. If a patient vomits after dosing on an empty stomach, give the next dose with food or a small treat; if vomiting continues, contact the veterinarian.