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By Wendy C. Brooks, DVM, DipABVP
Educational Director,

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Furosemide (Lasix, Salix, Disal)

Brand Name: Lasix, Salix, Disal

Available in 8 or 10 mg/ml oral solution; 12.5 mg, 20 mg, 40 mg, 50 mg & 80 mg tablets; and injectable

(The former brand name Lasix is now called Salix.)


The kidney is one of the most complicated organs of the body. It is responsible for maintaining electrolyte balance, blood pressure, and fluid/hydration status through its elaborate filtration and excretion systems. It is not our intention to review a system as complex as this here; with regard to furosemide, what is important to know is that it acts on the kidney to increase the body’s loss of water, assorted minerals, and assorted electrolytes (salt, potassium, calcium, magnesium, bicarbonate, hydrogen and ammonium).

How this Medication is Used

Because furosemide leads to water loss via increased urine production it is classified as a diuretic. This makes it a useful medication in conditions where the removal of excessive fluids would be beneficial. Such conditions include:

Congestive Heart Failure
By definition, congestive heart failure involves fluid congestion or accumulation somewhere where there should not be excess fluids. This accumulation is usually in the belly or chest cavity or actually within the lung tissue (pulmonary edema). Furosemide causes an increase in urine production, thus shifting the equilibrium away from the accumulation of fluid in tissue. Injectable furosemide is needed during acute heart failure crisis but oral is generally adequate for continued management after stabilization.

Furosemide can also be used to remove fluid from body cavities or peripheral tissues even when the cause is not heart failure. Swellings due to fluid accumulation can be reduced through using this medication.

Chronic Bronchitis
Furosemide can act as a respiratory airway dilator. Dilation is not only beneficial for patients coughing from congestive heart failure/pulmonary edema but also for patients coughing for other reasons. Dogs with chronic bronchitis can get relief from furosemide via airway dilation, which is completely independent of furosemide's diuretic effect.

Acute kidney failure
Another important reason to stimulate urine production could be acute kidney failure where the kidney is trying to shut down urine production permanently and fatally. Obviously this is a dire emergency and frequently a combination of diuretics is needed for treatment.

Furosemide can be used to reduce excessive blood calcium levels, which are inherently dangerous to the kidneys. Calcium is one of the minerals furosemide encourages the body to lose. Often prednisone is used with furosemide for this use as it too encourages calcium excretion.

Furosemide can also be helpful in reducing dangerously high potassium blood levels and has been used in the horse to treat exercise induced pulmonary hemorrhage (nose bleeds).

Side Effects

Inherent to all diuretics, including furosemide, are the following side effects:

  • Increased thirst
  • Increased urination

Furosemide is associated with an increase in blood sugar levels. This is not a problem for most patients but a different diuretic may be a better choice for a diabetic patient.

Hearing loss has been reported, especially in cats, when large amounts of injectable Lasix have been used in a crisis.

Urination is usually seen within 30 minutes of the administration of an oral dose of furosemide. The drug peaks in strength after 1 to 2 hours after it is given orally.

The chemical exchanges that furosemide exerts in the kidney involves release of potassium into the urine. It is possible that too much potassium loss can occur and the body can become potassium depleted. Weakness or lethargy could be an indicator that blood potassium has dropped too low. Be sure to inform your veterinarian if your pet seems listless.

Interactions with other Drugs

One of the most common drug interactions to be aware of is the interaction between furosemide and vasodilating heart medications (especially the angiotensin converting enzyme inhibitors such as enalapril, benazepril, and lisinopril). Furosemide will decrease circulating blood volume as it causes depletion in body water. This means that the kidney (and most other organs) will have a reduced blood flow to them. The patient’s kidney and electrolyte balance must stabilize before a vasodilator is added in. If the vasodilator is added too soon, it too will reduce blood flow to the kidney and the kidney will fail, possibly permanently.

These two types of medications are frequently used together in heart failure patients. To avoid problems, the patient should be assessed biochemically with a blood test prior to the addition of the vasodilator. If all is normal, the vasodilator may be started and renal blood parameters should be rechecked 3 to 5 days later. If they are still normal, then the above kidney issues should not be a problem; however, should another heart failure crisis arise and large doses of injectable Lasix be required to resolve it, kidney values should again be checked 3 to 5 days later.

The airway dilator theophylline may be able to reach higher blood levels when used in conjunction with furosemide. This means that the theophylline dose may need to be reduced.

Furosemide is often used in combination with prednisone to reduce serum calcium levels. It is possible for this combination of medication to lead to a reduction in potassium level significant enough to require potassium supplementation.

Aminoglycoside antibiotics (amikacin, gentamicin etc.) have properties that make them toxic to the ear and kidney. These properties increase with accompanying use of furosemide.

Concerns and Cautions

Obviously it is best not to use this medication in a dehydrated patient or if water is being restricted.

Because of the increased calcium excretion brought on by furosemide (i.e., an increase in urinary calcium levels), there could be a problem using this medication in patients with a history of calcium oxalate bladder stone formation.

It is extremely difficult to overdose with this medication. Toxic doses reported are over 100 times a typical oral dose of medication. It is important to realize that in the treatment of heart failure, which is this drug’s primary use, a crisis can arise at any time. Often giving an extra dose of oral medication can be a life-saving procedure. Be sure you understand what signs you are to watch for to determine if an extra dose or two should be administered and when you should definitely rush to the vet’s office.

The loss of water-soluble vitamins or urine can be a problem for patients receiving diuretics. Prescription diets for heart and kidney disease are vitamin-fortified with these extra losses in mind. Patients that refuse to eat such prescription diets may need a vitamin supplement.

Furosemide should be used with caution in diabetic patients as they are already prone to abnormalities in electrolyte balance, which could be exacerbated by the electrolyte shifts employed by furosemide.

It is our policy not to give dosing information over the Internet.


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