Lasix is a loop diuretic approved by the FDA for veterinary use. It contains the active ingredient furosemide which works by inhibiting the re-uptake of electrolytes, salt and fluid in the kidneys increasing the rate of urination while promoting the release of potassium. Its exact mechanism of action is not yet known. Vets often prescribe Lasix or generic furosemide for the treatment of uremia, hyperkalemia (excess potassium) and pulmonary edema amongst several other conditions.

During longer term treatment hydration and electrolyte levels should be monitored regularly. These levels should also be monitored closely when dogs being treated are suffering with vomiting or diarrhea.

What is a diuretic?

A diuretic is a drug which increases urinary output ridding the body of excess salt and water. Loop diuretics such as furosemide are quite powerful and inhibit the reabsorption of these substances from a certain part of the kidneys known as the “loop of Henle”.

Recommended Dosage

Note: Never begin treating your dog with Lasix (or with any other product containing furosemide) unless the vet has told you to do so.

The dosage of Lasix when used as a diuretic is often around 2 mg/lb once to twice daily. However, the dosage for this medicine is highly variable and is often started on the conservative side, then increased gradually until symptoms improve. A standard 2 mg/lb dose is shown on the chart below:

A chart displaying the diuretic dosage of furosemide for dogs

This dosage can increase or decrease considerably depending on the circumstances. The goal of treatment with Lasix is to only give the minimum required dose for adequate treatment. For this reason many vets start the dosage on the lower side and gradually increase it if needed.

Furosemide can be given orally with effects kicking in after one to two hours, or injected intravenously if needed. When injected the medicine begins working in as little as 5 minutes. In tablet form, Lasix can be bought in strengths of 20, 40 or 80 mg per pill but you can buy 12.5 mg generic furosemide pills if less is needed.

Treatment Of Medical Conditions

Please see the table below for typical dosages used in the treatment of various medical conditions:


Method of administration


Severe pulmonary edemaInjected (IV/IM)3.5 mg/lb per hour
Chronic heart failureOral0.5 mg/lb once daily
HyperkalemiaInjected (IV)0.9 mg/lb

Because the dosage of furosemide is highly variable your vet may give you a figure slightly higher or lower than those listed here depending on the severity of the condition. It may also be adjusted depending on existing medical conditions and other drugs your pet is taking.

Example: If your 40 lb dog suffers from heart failure, you may be prescribed 0.5 mg/lb once daily, which means you would administer 20 mg once a day.


How Safe Is It?

Lasix is safe and FDA-approved for animal use. The toxicity of the drug is low, which is partly due to its quick excretion from the body. During studies into the safety of furosemide for animals, dogs were given dosages of 2 mg/lb, 6 mg/lb and 10 mg/lb. After 9 days dogs receiving 10 mg/lb displayed only mild dehydration and fluctuations in blood and electrolyte levels. More on this study can be found here. Other studies into the prolonged use of the drug at doses of 4.5 mg/lb for six months found incidents of kidney scarring and calcification.

There are several contraindications (including anuria) and drug interactions associated with the drug and it has been shown to cause abnormalities during pregnancy.

FDA Status: The medicine is approved by the FDA in pill, syrup and injectable form. See application numbers 034-478, 034-621 and 102-380.

Guidelines For Use

Furosemide should only be used when it has been prescribed by a vet. Tell your vet about any medical conditions your dog suffers from, particularly any related to urinary output, as well as any medicine they are taking. Once your pet is deemed suitable and treatment begins you should:

  • Provide your dog with plenty of water
  • Provide adequate amounts of food

Because Lasix will cause the loss of electrolytes and water it’s essential that you keep this under control by providing plenty of water and food to replenish what is lost. Dogs suffering from gastric effects such as vomiting and diarrhea, or those undergoing prolonged treatment will need to be monitored very closely to ensure these levels are kept under control.

According to the manufacturer, treatment should be discontinued if dogs with progressive kidney disease display signs of increased blood nitrogen levels or decreased urinary output.

Note: If the frequency of urination slows down or stops completely after administering furosemide call the vet immediately.

How Can It Help Dogs?

Lasix is often given to dogs to prevent the build up of fluid in the respiratory and circulatory system. Conditions which may benefit from treatment includes:

  • Hyperkalemia
  • Pulmonary edema
  • Cardiogenic edema
  • Chronic heart failure
  • Nephrocalcinosis
  • Uremia

It is also used to prevent high blood pressure (hypertension) though drugs like enalapril are more commonly used for this purpose.

Side Effects

During treatment the following side effects may occur:


  • Increased urinary output
  • Increased thirst
  • Increased appetite


  • Headache
  • Drowsiness
  • Cough
  • Fever
  • Shortness of breath

Incidence unknown *

  • Dehydration
  • Hyponatremia
  • Hypokalemia
  • Hypomagnesemia
  • Hypocalcemia

* These symptoms are usually controlled by supplying adequate amounts of food and water during treatment.

Call the vet about any symptoms your pet finds bothersome. Call for immediate care if serious side effects occur including shortness of breath or an allergic reaction.


Overdoses of Lasix can be life threatening. The LD50 is around 454 mg/lb orally, and 136 mg/lb when injected. Symptoms of overdose includes dehydration, electrolyte imbalance, coma, seizures and circulatory collapse. If your pet has ingested an overdose by mouth, or if an overdose has been injected, emergency veterinary attention is required. Your dog’s electrolyte and water levels will be aggressively monitored along with their vital signs. Your pet’s gut may be emptied if the pills were taken recently.