Warfarin

What is warfarin?

Warfarin (brand names: Coumadin®, Jantoven®, Panwarfin®) is an anticoagulant used to prevent abnormal clotting due to conditions such as heart disease. It is usually only used in dogs, as the benefits in cats and other species have not been established.

Its use in dogs and occasionally horses to treat abnormal clotting is off label (extra label). Many drugs are commonly prescribed for off-label use in veterinary medicine. “Off label” means the drug is being used in a way that has not been reviewed or approved by the U.S. Food and Drug Administration (FDA) or Health Canada. In these instances, follow your veterinarian’s directions and cautions carefully, as they may be significantly different from those on the label.

How is warfarin given?

• Warfarin is given by mouth in the form of a tablet. It can also be compounded into a liquid form.
• Shake liquid forms well and measure very carefully.
• Do not cut tablets unless the tablet is scored (has a groove). If you cut a tablet that is not scored, it may result in uneven dosing.
• Warfarin may be given with or without food. However, if vomiting occurs when given on an empty stomach, give future doses with food.
• While using this medication, pets should not participate in activities that can lead to bleeding or other injuries.

Warfarin is considered a hazardous drug, so it is essential to avoid accidental exposure. Wear gloves when handling the medication and do not handle it if you are pregnant or nursing. If your pet vomits after receiving this drug, wear gloves while cleaning up.

This medication should take effect within 1 to 2 hours; however, effects may not be noted outwardly, so laboratory tests may be needed to evaluate this medication’s effectiveness.

What if I miss giving my pet the medication?

If you miss a dose, give the next dose as soon as you remember, and then continue with the regular dosing schedule. However, if it is less than 12 hours until the next scheduled dose, either:

• Skip the dose you missed, give it at the next scheduled time, and continue with the regular dosing schedule, OR
• Give the missed dose and then wait the recommended interval before giving the next dose (continue giving it regularly at that new time).

Are there any potential side effects?

Serious adverse reactions associated with this drug may occur without warning and could, in rare cases, lead to death. These signs are associated with abnormal bleeding and indicate the need for a dose adjustment. If any of the following signs develop, stop giving warfarin and contact your veterinarian immediately:

• Pale gums
• Weakness
• Nose bleeds
• Swollen areas around the body
• Blood in the vomit, urine, or stool
• Collapse

Other side effects may occur. If you notice anything unusual, contact your veterinarian immediately.  This short-acting medication should stop working within 24 hours, although effects can be longer in pets with liver or kidney disease.

Are there any risk factors for this medication?

DO NOT USE warfarin in:

• Pets that are allergic to the medication
• Pets with bleeding diseases
• Pets experiencing active bleeding
• Pets about to undergo certain surgical procedures
• Pets with an aneurysm, acute kidney disease, brain bleeds, blood cell diseases, uncontrolled high blood pressure, liver disease, and certain types of carcinomas (unless the benefits outweigh the risks)
• Pets that are pregnant

USE WITH EXTREME CAUTION in:

• Cats
• Pets that are nursing (their infants should be monitored for any signs of abnormal bleeding or bruising)

Are there any drug interactions I should be aware of?

The following medications should be used with caution when given with warfarin, as dose adjustments may be necessary:

• Acetaminophen
• Allopurinol
• Amiodarone
• Antibiotics
• Antidepressants
• Antifungals
• Barbiturates
• Blood thinners/anticoagulants
• Cisapride
• Corticosteroids
• Cyclosporine
• Fish oil
• Glucosamine
• Lactulose
• Nonsteroidal anti-inflammatory drugs (NSAIDs)
• Pentoxifylline
• Proton Pump Inhibitors (PPIs)
• Spironolactone
• Sucralfate
• Thyroid medications
• Tramadol
• Vitamin E and K

Many medications can increase or decrease warfarin’s efficacy, so be sure to tell your veterinarian about any medications (including vitamins, supplements, and herbal therapies) that your pet is taking before starting any new treatment.

Is any monitoring necessary with this medication?

• Frequent monitoring is recommended, particularly when first starting this medication. Monitoring may include a PT clotting test, INR blood test (international normalized ratio), platelet counts, blood cell concentrations, stool checks, and urinalysis.
• Your veterinarian will let you know when and how they want to reassess your pet to ensure the medication is working and not causing side effects.
• At home, monitor for any side effects such as those noted above. If any occur, stop using the medication and contact your veterinarian.

How do I store warfarin?

• Store this medication at room temperature, less than 104°F (40°C) and keep it away from light.
• Store compounded forms of this medication according to the label.
• Keep the container out of reach of children and pets.

What should I do in case of emergency?

If you suspect an overdose or an adverse reaction to the medication, call your veterinary office immediately. If they are not available, follow their directions in contacting an emergency facility. Be prepared to provide information about the product and your pet's weight and symptoms. Signs of an overdose/toxicity can be delayed and can occur up to 5 days later.