A clinical guide to furosemide (Salix) in dogs and cats: loop diuretic mechanisms, the ACVIM Stage C protocol, kidney monitoring, feline ototoxicity, and openFDA reports.

Congestive Heart Failure (CHF) is one of the most common and life-threatening chronic conditions diagnosed in older dogs and cats. Whether caused by myxomatous mitral valve disease (MMVD) or dilated cardiomyopathy (DCM) in dogs, or hypertrophic cardiomyopathy (HCM) in cats, the clinical result of heart failure is the same: the heart can no longer pump blood efficiently, leading to a backup of pressure in the venous system and the accumulation of fluid in the lungs (pulmonary edema) or the abdomen (ascites).

Eliminating this fluid and preventing its recurrence is the most critical therapeutic goal in CHF management. To achieve this, veterinarians rely on furosemide (commonly known by the brand names Salix, Lasix, or Disal) as the cornerstone diuretic. Furosemide is highly effective, but it is not a standalone solution. It requires integration into a multi-drug protocol, and its use is a balancing act between clearing pulmonary congestion and preserving kidney function. Because it causes significant excretion of water and minerals, regular kidney and electrolyte monitoring is mandatory to prevent severe dehydration, renal failure, and electrolyte imbalances.

This guide provides a comprehensive clinical overview of furosemide for dogs and cats. We examine its pharmacology, approved products, role in heart failure staging, clinical monitoring and side effects, drug-drug interactions, and an analysis of the FDA's veterinary adverse-event reports.

Mechanism of Action: The Loop Diuretic

Furosemide belongs to the loop diuretic class of medications. It is a sulfonamide derivative that targets the kidneys to promote the excretion of sodium, chloride, and water.

Cellular Mechanism of Action

Furosemide operates in the thick ascending limb of the loop of Henle, which is responsible for reabsorbing approximately 25% of the filtered sodium load from the urine back into the blood. Furosemide enters the renal tubule via active secretion in the proximal tubule and travels to the loop of Henle. Once there, it binds reversibly to the chloride-binding site of the Na⁺-K⁺-2Cl⁻ (NKCC2) cotransporter on the apical membrane of the tubular epithelial cells.

By blocking this cotransporter, furosemide inhibits the active reabsorption of sodium, potassium, and chloride. Because water follows sodium passively, this blockade causes a massive volume of water to remain within the renal tubule, leading to profuse urine production (diuresis).

Furthermore, the inhibition of sodium and chloride reabsorption disrupts the electrical gradient across the tubular membrane, which in turn prevents the passive reabsorption of other critical divalent cations, specifically calcium (Ca²⁺) and magnesium (Mg²⁺). As a result, furosemide administration increases the renal excretion of:

Sodium (Na⁺)
Chloride (Cl⁻)
Potassium (K⁺)
Hydrogen ions (H⁺)
Ammonium (NH₄⁺)
Calcium (Ca²⁺)
Magnesium (Mg²⁺)
Water

This rapid, high-ceiling diuresis reduces blood volume, lowers systemic venous pressures, and allows the lymphatic system to clear fluid from the lungs, relieving the clinical signs of congestive heart failure.

Approved Products and Labeled Indications

Veterinarians use both injectable and oral formulations of furosemide to manage heart failure.

FDA-Approved Products

Salix Injection (NADA 034-700): Manufactured by Merck Animal Health, this is a 5% (50 mg/mL) sterile solution of furosemide. It is FDA-approved for use in dogs, cats, and horses via intramuscular (IM) or intravenous (IV) injection.
Furosemide Tablets (ANADA 200-759): There are several veterinary generic oral tablet formulations (available in 12.5 mg and 50 mg sizes) that are FDA-approved for dogs and cats.

Labeled Indications

The FDA-approved labels specify that furosemide is indicated for the treatment of edema (pulmonary congestion, ascites) associated with cardiac insufficiency and acute non-inflammatory tissue edema in dogs and cats.

Dosing Guidelines

Labeled Dose: The approved product label for Salix injection and furosemide tablets carries a recommended dose of 1 to 2 mg per pound (approximately 2.5 to 5 mg/kg) once or twice daily by mouth, IM, or IV.
Species Variance: The label suggests that the dose should be adjusted based on the severity of the edema and the patient's response, and notes that a lower dosage range is generally sufficient for cats.
Veterinary Supervision: Dose adjustments must be made under the direct supervision of a veterinarian. The dose is titrated to the lowest effective level that keeps the patient free of pulmonary congestion, as over-diuresis can lead to prerenal azotemia and severe electrolyte depletion.

Heart Failure Staging and the ACVIM MMVD Staging System

Furosemide's clinical utility is defined by the patient’s stage of heart disease. The American College of Veterinary Internal Medicine (ACVIM) Consensus Guidelines for myxomatous mitral valve disease (most recently updated in 2019) outline how diuretics are integrated into heart failure management.

The ACVIM Staging System

The stages run from A (at-risk, no disease) through B1 (murmur, no enlargement) and B2 (murmur plus cardiac enlargement) to C (past or current congestive heart failure) and D (refractory heart failure). Furosemide enters the protocol at Stage C; it is not used in Stage A or B.

Stage A: Dogs at high risk of developing heart disease (e.g., Cavalier King Charles Spaniels) but with no murmur or structural abnormalities. No drug therapy is indicated.
Stage B1: Dogs with a heart murmur but no radiographic or echocardiographic evidence of cardiac enlargement. No drug therapy is indicated.
Stage B2: Dogs with a heart murmur and clear structural cardiac enlargement (cardiomegaly), but who have never developed clinical signs of heart failure. The landmark EPIC trial demonstrated that administering pimobendan (Vetmedin) at this stage delays the onset of heart failure by an average of 15 months. Furosemide is NOT used in Stage B2. Using a loop diuretic prior to the onset of congestion unnecessarily activates the renin-angiotensin-aldosterone system (RAAS) and can cause premature renal injury.
Stage C: Dogs with past or current clinical signs of congestive heart failure (pulmonary edema or ascites) secondary to structural heart disease. Stage C is the threshold where furosemide becomes mandatory.
Stage D: Dogs with end-stage, refractory congestive heart failure whose clinical signs are no longer controlled by standard Stage C therapy. These patients require advanced, higher-dose diuretic strategies.

Clinical Protocols in Stage C and Stage D

Acute Stage C (Congestive Crisis)

When a dog or cat presents in respiratory distress due to acute pulmonary edema, it represents a medical emergency. The first-line protocol is:

High-Dose Injectable Furosemide: Administered at 2 to 4 mg/kg IV or IM every 1 to 2 hours, or as a constant rate infusion (CRI) at 0.66 to 1 mg/kg/hour, until the respiratory rate decreases and the patient stabilizes.
Supportive Care: Oxygen therapy, minimal handling, and mild sedation (often with butorphanol) to reduce anxiety.

Chronic Stage C (Home Maintenance)

Once the patient is stable and the congestion is cleared, they transition to oral maintenance therapy. Furosemide must never be used as a standalone treatment. The standard ACVIM Stage C maintenance protocol (often called the "Quad Therapy") includes:

Furosemide: Dosed at 1 to 2 mg/kg PO every 12 hours, titrated to the lowest effective dose.
Pimobendan (Vetmedin): An inodilator that increases cardiac contractility and causes vasodilation, reducing the heart's workload. Dosed at 0.25–0.3 mg/kg PO BID. (For details on pimobendan dosing and the EPIC trial data, see the vetmedin for dogs monograph.)
ACE Inhibitor (e.g., Enalapril or Benazepril): Blocks the conversion of angiotensin I to angiotensin II, preventing vasoconstriction and aldosterone release. Dosed at 0.5 mg/kg PO BID or daily.
Spironolactone: A weak, potassium-sparing aldactone-antagonist diuretic that provides cardioprotective effects by blocking aldosterone-mediated myocardial fibrosis. Dosed at 1–2 mg/kg PO daily.

Refractory Stage D (Advanced Therapy)

When the heart failure becomes refractory to standard dosing, several advanced strategies may be utilized:

Dose Escalation: Increasing the furosemide dose up to 4 mg/kg PO every 8 hours.
Sequential Nephron Blockade: Adding a thiazide diuretic (such as hydrochlorothiazide) to block sodium reabsorption in the distal convoluted tubule. This synergizes with furosemide, which blocks the loop of Henle, but it carries a very high risk of triggering acute kidney injury and must be monitored closely.
Loop Diuretic Switch: In some dogs, switching from furosemide to torsemide—a newer, more potent loop diuretic with longer bioavailability and anti-aldosterone properties—can restore diuresis and extend survival. Torsemide is typically dosed at 5% to 10% of the patient's total daily furosemide dose once daily.

For a comprehensive review of heart failure staging, diagnostic workups, and emergency protocols, see the congestive heart failure in dogs condition guide.

The Monitoring Protocol: Preserving Kidneys and Electrolytes

Because furosemide works by forcing the kidneys to excrete water and minerals, it places a significant burden on the renal system. Veterinary teams must establish a structured monitoring schedule to balance cardiac clearance with renal safety.

The Key Renal and Electrolyte Risks

Azotemia (Prerenal vs. Renal): As furosemide reduces blood volume to clear pulmonary edema, it also decreases renal perfusion. This leads to a rise in blood urea nitrogen (BUN) and creatinine (azotemia). A mild rise in renal values is often an expected trade-off to keep the lungs clear, but a sudden, severe elevation indicates prerenal dehydration or active kidney damage.
Hypokalemia (Low Potassium): Furosemide causes massive potassium excretion. Potassium is vital for cardiac electrical activity and muscle contraction. Hypokalemia can cause muscle weakness, lethargy, and cardiac arrhythmias, and it significantly increases the risk of digoxin toxicity.
Dehydration: Over-diuresis depletes intracellular and extracellular fluid volume, leading to dry mucous membranes, prolonged skin tenting, and hypotension.
Hypomagnesemia: Magnesium excretion is linked to calcium and potassium excretion. Low magnesium can worsen hypokalemia and increase cardiac irritability.

The Recheck Monitoring Schedule

Veterinary practices should implement a standard recheck protocol following the initiation of, or any change to, furosemide therapy:

First recheck: 7 to 14 days after starting or adjusting the dose. Run a renal chemistry panel (BUN, creatinine, phosphorus), an electrolyte panel (sodium, potassium, chloride), a blood-pressure reading, and assess the resting respiratory rate and hydration status.
If renal values are stable, continue the current dose and recheck again in 3 to 6 months, sooner if clinical signs change.
If renal values have risen or electrolytes have shifted, adjust the dose, address hydration (subcutaneous fluids if prerenal), and recheck again in about 7 days rather than waiting for the next routine interval.

At each recheck visit, the veterinary team must evaluate:

Renal Chemistry Panel: Specifically monitoring BUN, creatinine, and phosphorus.
Electrolyte Panel: Measuring sodium (Na⁺), potassium (K⁺), and chloride (Cl⁻). If potassium levels drop below 3.5 mEq/L, oral potassium gluconate supplementation should be started, or the spironolactone dose adjusted.
Blood Pressure: Monitoring for systemic hypotension secondary to volume depletion.
Hydration Status: Assessing mucous membranes, capillary refill time (CRT), and body weight (a rapid loss of weight often reflects acute dehydration rather than fat loss).
Sleeping Respiratory Rate (SRR) Log: Owners should count the pet’s breaths per minute while sleeping at home. An SRR consistently under 30 breaths per minute indicates the heart failure is well-controlled. A rising SRR is the earliest indicator that pulmonary edema is returning, signaling that a dose increase may be needed before a congestive crisis occurs. For clinical staging and diagnostic examinations that precede CHF, see our guide on evaluating a heart murmur in dogs.

Feline-Specific Cautions: Dehydration and Ototoxicity

Feline patients require careful management when receiving loop diuretics. They are far more sensitive to the side effects of furosemide than dogs.

High Dehydration Risk

Cats have highly concentrated urine and a delicate fluid balance. They are prone to rapid dehydration and prerenal azotemia when given loop diuretics. This risk is amplified because many older cats have pre-existing, undiagnosed chronic kidney disease (CKD).

When furosemide is administered, it can accelerate the progression of renal failure. The maintenance dose in cats should be titrated to the lowest possible frequency (often 1 mg/kg once daily or every other day) that prevents pulmonary edema. For renal staging and dietary management in cats, see the feline CKD treatment guide.

Ototoxicity and Hearing Loss

Loop diuretics can cause transient or permanent ototoxicity (hearing loss and vestibular dysfunction) in both species, but cats are particularly sensitive.

The NKCC2 cotransporter targeted by furosemide in the kidney is structurally similar to the NKCC1 cotransporter located in the stria vascularis of the inner ear. The stria vascularis produces endolymph, the fluid that fills the membranous labyrinth. By blocking this transporter in the ear, high doses of loop diuretics alter the ionic composition of the endolymph, leading to a loss of endocochlear potential. This causes:

Hearing impairment or total deafness.
Vestibular ataxia (wobbly gait, head tilt, loss of balance).
Nystagmus (rapid, involuntary eye movement).

Ototoxicity is typically associated with rapid intravenous administration of high doses during acute congestive crises, or when oral doses are escalated in advanced disease. To minimize this risk:

Administer intravenous injections slowly over several minutes rather than as a rapid bolus.
Avoid the concurrent use of other ototoxic drugs.
Watch cats closely for changes in hearing or balance during therapy.

High-Risk Drug Interactions

Furosemide’s effects on fluid volume and electrolytes contribute to several serious drug-drug interactions:

1. Aminoglycoside Antibiotics (The Ototoxicity and Nephrotoxicity Trap)

Aminoglycosides (such as gentamicin, amikacin, and neomycin) are highly effective against severe Gram-negative infections, but they are inherently nephrotoxic and ototoxic.

Administering furosemide alongside aminoglycosides drastically increases these risks. The diuretic-induced dehydration reduces renal clearance of the antibiotic, leading to higher, toxic blood levels. Simultaneously, both drugs target the structures of the inner ear, compounding the risk of permanent deafness and vestibular damage. Co-administration of these drugs should be avoided.

2. Corticosteroids

Glucocorticoids (such as prednisone, prednisolone, and dexamethasone) promote potassium excretion in the distal tubules. When given concurrently with furosemide, the potassium-wasting effects are additive, significantly increasing the risk of severe hypokalemia.

3. Digoxin and Digitalis Glycosides (The Digoxin Toxicity Trap)

Digoxin is sometimes used in veterinary medicine to control the heart rate in dogs with dilated cardiomyopathy or atrial fibrillation. The pharmacological activity of digoxin is highly dependent on serum potassium levels; potassium normally competes with digoxin for binding sites on the Na⁺-K⁺-ATPase pump.

If furosemide causes hypokalemia, the lack of competing potassium allows digoxin to bind to the pump in excess, significantly increasing the risk of digoxin toxicity, even if digoxin blood levels are within the normal reference range. Symptoms of digoxin toxicity include severe vomiting, diarrhea, anorexia, depression, and life-threatening cardiac arrhythmias. Digoxin levels and electrolytes must be checked regularly in patients receiving both drugs.

4. NSAIDs

Non-steroidal anti-inflammatory drugs (NSAIDs, such as carprofen, meloxicam, or galliprant) inhibit prostaglandin synthesis in the kidneys. Prostaglandins are essential to maintain renal blood flow via vasodilation, especially in patients with heart failure where renal perfusion is already compromised.

Administering NSAIDs to an animal receiving furosemide can cause acute renal failure by blocking this vasodilatory mechanism, and the NSAID can reduce the diuretic efficacy of the furosemide.

5. Antidiabetic Drugs

Furosemide can cause transient hyperglycemia and decrease glucose tolerance, which may reduce the efficacy of insulin or oral hypoglycemic agents in diabetic patients. Diabetic dogs and cats started on furosemide should have their blood glucose levels monitored closely.

Analysis of FDA openFDA Adverse-Event Data

To evaluate the safety profile of furosemide in real-world clinical practice, we analyzed the FDA Center for Veterinary Medicine's animal drug adverse-event records, accessed through the openFDA database (analysis run date: 2026-07-05).

The search identified 2,485 unique reports where furosemide was recorded as an active ingredient. The species breakdown shows a strong canine focus, reflecting the high prevalence of canine congestive heart failure:

Dogs: 1,477 reports (59.4%)
Cats: 282 reports (11.3%)
Other (Horses, Cattle, Humans, Unknown): 726 reports (29.2%)

Top Reported Reaction Terms

The table below lists the most frequent clinical reaction terms associated with furosemide in the database:

Position	Reaction Term	Report Count	Clinical Context and Interpretation
1	Vomiting	293	Common gastrointestinal side effect; direct gastric irritation.
2	Death by euthanasia	256	Outcome; reflects humane endpoints in end-stage heart or kidney disease.
3	Death	253	Outcome; mortality associated with congestive heart failure.
4	Lethargy	252	Systemic sedation; heavily confounded by heart failure and dehydration.
5	Diarrhoea	223	Gastrointestinal disruption; direct mucosal irritation.
6	Anorexia	172	Decreased appetite; common secondary sign of nausea or congestion.
7	Elevated blood urea nitrogen (BUN)	150	Direct renal signal; reflects prerenal azotemia and diuretic workload.
8	Seizure	146	Neurological sign; often linked to severe electrolyte shifts (hyponatremia, hypokalemia).
9	Cough	138	Cardiac sign; represents active pulmonary congestion from heart failure.
10	Abnormal radiograph finding	136	Diagnostic marker; represents cardiomegaly or active pulmonary edema.

(Note: The raw database also returned "Color, Abnormal" at 355 reports and "Solution, Abnormal" at 148 reports. These are product quality defect markers—typically reporting crystallization or discoloration in injectable vials—and are not clinical patient reactions.)

Interpretation of Safety Signals

A detailed analysis of these reported reactions reveals important clinical lessons:

The Cardiac and Renal Disease Population Confound: The database is dominated by outcomes like "Death by euthanasia" (256), "Death" (253), and clinical markers like "Elevated BUN" (150), "Cough" (138), and "Abnormal radiograph finding" (136). This signature is a direct reflection of the sick target population. Dogs and cats receiving furosemide have severe, chronic cardiovascular disease and are prone to renal impairment. When a dog dies of heart failure or is euthanized because of refractory pulmonary edema, furosemide is listed on the adverse event report because it was part of their daily medication regimen. These counts represent passive surveillance data and must not be interpreted as evidence that furosemide caused the deaths or the underlying heart failure.
Gastrointestinal and Renal Warnings: The frequency of vomiting (293), anorexia (172), and elevated BUN (150) directly supports the drug’s labeled safety warnings. It emphasizes the need for regular kidney blood work, hydration assessments, and clinical monitoring at each recheck visit.
Seizures and Electrolyte Shifts: The presence of seizures (146 reports) is a clinical warning. Severe electrolyte abnormalities—specifically profound hyponatremia (low sodium) or hypokalemia—can cause cerebral edema and neurological signs, including muscle tremors and seizures. This highlights the importance of maintaining the lowest effective diuretic dose and avoiding sudden, unmonitored dose increases.

Frequently Asked Questions

Why is my dog or cat on furosemide for heart disease, and how long will they need it?

Furosemide is a diuretic that helps clear fluid backup (pulmonary edema or ascites) caused by congestive heart failure. It works by telling the kidneys to excrete extra water and salt into the urine. Because heart disease is chronic and progressive, your pet will need to take furosemide for the rest of their life to keep the fluid from returning to their lungs.

What side effects of furosemide mean I should call the vet sooner than the recheck?

You should contact your veterinarian immediately if you notice signs of severe dehydration or kidney strain, including:

Profound lethargy or weakness.
Complete loss of appetite (anorexia).
Persistent vomiting or diarrhea.
A sudden increase in breathing rate or effort at rest (sleeping respiratory rate over 30 breaths per minute).
Weakness in the hind legs or a wobbly gait (often indicating low potassium).

Why is potassium or spironolactone added to furosemide?

Furosemide causes the kidneys to excrete large amounts of potassium. If potassium levels drop too low, it can cause muscle weakness and heart arrhythmias. To prevent this, veterinarians often prescribe a potassium supplement, or add spironolactone to the treatment plan. Spironolactone is a weak diuretic that blocks the hormone aldosterone and helps the body retain potassium while providing protective benefits for the heart muscle.

Is furosemide safe for cats, and why is the dose lower?

Furosemide is safe for cats when used carefully, but cats are highly sensitive to dehydration, kidney strain, and ototoxicity (hearing loss and balance issues associated with high doses of loop diuretics). Because of this sensitivity, cats are typically started on the lower end of the dosing range and monitored closely with regular blood work to check their kidney values and electrolytes.

Sources

FDA CVM. ANADA 200-759 (furosemide tablets for dogs and cats) approval and Freedom of Information summary. https://animaldrugsatfda.fda.gov/adafda/app/search/public/document/downloadFoi/17256
DailyMed. Salix (furosemide injection) full prescribing label. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=c3ff7e14-8232-40df-a5c8-60f98f5c1025
PubMed Central (PMC). ACVIM consensus guidelines for the diagnosis and treatment of myxomatous mitral valve disease in dogs (PMC6524084). https://pmc.ncbi.nlm.nih.gov/articles/PMC6524084/
Merck Veterinary Manual. Diuretics: Pharmacology and Clinical Use in Animals. https://www.merckvetmanual.com/pharmacology/systemic-pharmacotherapeutics-of-the-cardiovascular-system/diuretics-for-use-in-animals
Merck Animal Health. Salix injection product information. https://www.merck-animal-health-usa.com/products/salix-furosemide-injection/
Veterinary Information Network (VIN) / Veterinary Partner. Furosemide Monograph. https://veterinarypartner.vin.com/default.aspx?pid=19239&catId=102894&id=4951415
Plumb's Veterinary Drug Handbook. Furosemide monograph. https://www.plumbs.com/
FDA Center for Veterinary Medicine. openFDA Animal & Veterinary Adverse Event API (report-level adverse-event records, 1987–2026; analysis run date 2026-07-05). https://open.fda.gov/apis/animalandveterinary/

Vet Med Guide

Furosemide (Salix) for Dogs and Cats: CHF Diuretic, Electrolytes, and the FDA Data