By Rania Gollakner, BS, DVM, MPH

What is warfarin?

Warfarin (brand names: CoumadinJantovenPanwarfin) 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 either ‘off label’ or ‘extra-label’. Many drugs are commonly prescribed for off label use in veterinary medicine. In these instances, follow your veterinarian’s directions and cautions very carefully as their direction 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. It may be given with or without food. However, if vomiting occurs when given on an empty stomach, future doses should be given with food. Liquid forms should be shaken well and measured very carefully. Tablets should not be cut unless the tablet is scored (has a groove). Tablets that are not scored, may result in uneven dosing when cut.  

This medication should take effect within 1 to 2 hours, however, effects may not be noted outwardly and therefore laboratory tests may need to be done to evaluate this medication’s efficacy.  

What if I miss giving my pet the medication? 

If you miss giving your pet a dose, give the next dose as soon as you remember, but if it is closer than 12 hours before 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? 

The most common side effect is abnormal bleeding. Signs of abnormal bleeding include pale gums, weakness, nose bleeds, swollen areas around the body, blood in the vomit, urine, or stool, or more severely, collapse and death. Signs of abnormal bleeding indicate the need for a dose adjustment. Contact your veterinary clinic if these symptoms occur.

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, or in pets about to undergo certain surgical procedures. Ideally, warfarin should be avoided in pets with an aneurysm, acute kidney disease, brain bleeds, blood cell diseases, uncontrolled high blood pressure, liver disease and certain types of carcinoma. Warfarin should be used with extreme caution, if at all, in cats. Pets should not participate in activities that can lead to bleeding or other injuries while using this medication. 

Warfarin should not be given to pets that are pregnanand pregnant humans administering this medication, should use gloves and avoid inhaling any tablet dust. It should be used cautiously in pets that are nursing and the 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, anabolic steroids, antibiotics, antidepressants, antifungals, barbiturates, blood thinners/anticoagulantscisapridecorticosteroids, cyclosporinefish oil, glucosamine, hormones, lactulose, mercaptopurine, NSAIDs, pentoxifylline, phenylbutazone, PPIs, quinidine, salicylates, spironolactone, sucralfate, thyroid medications, tramadol and vitamin E and K.

Your veterinarian should be advised of any medications (including vitamins, supplements, or herbal therapies) that your pet is taking, as many medications can increase or decrease warfarin’s efficacy.  

Is there any monitoring that needs to be done 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 monitor your pet to be sure that the medication is working and you should monitor your pet at home for any side effects. 

How do I store warfarin? 

Store this medication at room temperature, less than 40°C (104°F) and keep it away from light. For compounded forms of the medication, please follow the storage instructions on the label. 

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. Signs of an overdose/toxicity can be delayed and can occur up to 5 days later.  

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