Furosemide (Lasix) for Cats: Uses, Dosage & Side Effects

This information is for educational purposes only. Never give your pet any medication without your veterinarian's guidance. Dosing, frequency, and safety depend on your pet's specific health profile.

Furosemide, often called Lasix, is a prescription loop diuretic. That means it helps the kidneys move more salt and water into the urine. In cats, your vet most often uses it when fluid buildup is making breathing harder or putting extra strain on the body.

In practical terms, furosemide helps remove excess fluid from the lungs or other tissues. It does not cure the underlying heart or kidney problem, but it can be very important for symptom control and comfort. Many cats with heart disease receive it as part of a larger treatment plan.

Cats can be more sensitive to furosemide than dogs, so dosing and monitoring matter. Your vet may adjust the amount over time based on breathing rate, hydration, kidney values, electrolytes, and how well your cat is responding at home.

In cats, furosemide is used most commonly to manage congestive heart failure, especially when fluid backs up into the lungs and causes pulmonary edema. Cats with heart disease may show fast breathing, increased effort to breathe, lethargy, or open-mouth breathing. If your cat is struggling to breathe, see your vet immediately.

Your vet may also use furosemide for other fluid-related problems, including some cases of body cavity fluid buildup or selected kidney-related conditions where removing excess fluid is helpful. In emergency settings, injectable furosemide may be used to quickly reduce life-threatening fluid overload.

Because furosemide treats the fluid consequence of disease rather than the root cause, it is often paired with other medications and follow-up testing. Depending on the diagnosis, your vet may combine it with heart medications, blood pressure medications, or other diuretics.

Furosemide dosing in cats is individualized. For long-term management of pulmonary edema, veterinary references note that many cats need about 1-2 mg/kg by mouth every 12-24 hours, while emergency injectable dosing may be around 0.5-2 mg/kg every 1-8 hours depending on the situation and route. Some cats with severe heart failure need higher doses, but that increases the need for close monitoring.

Your vet may prescribe tablets, liquid, or an injectable form used in the hospital. Never change the dose, frequency, or formulation on your own. A dose that is too low may not control fluid buildup. A dose that is too high can raise the risk of dehydration, kidney injury, and electrolyte problems.

At home, your vet may ask you to track your cat's resting breathing rate, appetite, water intake, energy level, and urination. Recheck exams and bloodwork are often part of safe treatment, especially early on or after a dose change. If your cat seems weak, stops eating, vomits repeatedly, or breathes worse instead of better, contact your vet right away.

The most expected effects of furosemide are increased urination and often increased thirst. Those changes happen because the medication is pulling extra fluid out of the body. Mild appetite changes or stomach upset can also occur in some cats.

More serious side effects are usually related to too much fluid loss or electrolyte imbalance. Warning signs can include lethargy, weakness, dehydration, dry or tacky gums, poor appetite, vomiting, diarrhea, collapse, or worsening kidney values on lab work. Cats that do not have easy access to water are at higher risk of dehydration.

Rarely, furosemide can contribute to hearing-related toxicity, especially at very high doses or when combined with other ototoxic drugs. Contact your vet promptly if your cat seems disoriented, unusually weak, stops producing normal urine, or has any worsening breathing signs. Breathing distress is an emergency.

Furosemide can interact with several other medications, which is one reason your vet should know everything your cat receives, including supplements and any human medications in the home. Important concerns include drugs that can also affect the kidneys, blood pressure, hydration status, or electrolyte balance.

Veterinary references specifically warn that furosemide can increase the nephrotoxic and ototoxic risk of aminoglycoside antibiotics. It should also be used carefully with other diuretics, some heart medications, and drugs that may lower blood pressure or change potassium levels. NSAIDs can also complicate kidney perfusion and should never be added unless your vet directs it.

If your cat is on chronic furosemide, ask before starting anything new. That includes over-the-counter products, compounded medications, appetite aids, and supplements. The safest plan is coordinated prescribing with your vet so the whole medication list is reviewed together.