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Warfarin Sodium (Coumadin®)

By: Dr. Dawn Ruben

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  • Normally the body maintains a fine balance between the formation of blood clots and the breakdown of clots. Inadequate clotting can lead to some serious bleeding problems.
  • Conversely, excessive blood clotting can clog arteries or veins, leading to obstruction of blood flow to internal organs, and then damage to organ tissue.
  • Furthermore, blood clots that form in one area, such as the heart or a vein, can be carried by the blood to other parts of the body (such as the brain, lungs or limbs). A clot that starts in one area and then travels to another is called an embolus (or thromboembolus) and the situation is called thromboembolic disease.
  • Examples of thromboembolic disease include pulmonary embolism (clot in the lung), aortic embolism (clot at the origin of the back legs) and stroke (clot in the brain).
  • Preventing these type of blood clots from growing or forming is important in certain medical conditions. Warfarin belongs to a class of drugs known as coumarin derivatives and is an important anti-clotting drug used in medicine.
  • Warfarin acts by interfering with Vitamin K, a vitamin crucial in the synthesis of certain clotting proteins.
  • Warfarin is a prescription drug and can only be obtained from a veterinarian or by prescription from a veterinarian.
  • This drug is not approved for use in animals by the Food and Drug Administration but it is prescribed legally by veterinarians as an extra-label drug.

    Name Brands and Other Brands

  • This drug is registered for use in humans only.
  • Human formulations: Coumadin® (DuPont)
  • Veterinary formulations: None

    Uses of Warfarin

  • Warfarin is used primarily to treat and prevent blood clots.
  • It is administered to cats with cardiomyopathy to prevent formation of clots in the left atrial chamber of the heart. These clots often break off and obstruct blood flow to the back legs.
  • It can be used in dogs with evidence of pulmonary embolism (lung clots) to prevent further blood clotting.

    Precautions and Side Effects

  • While generally safe and effective when prescribed by a veterinarian, warfarin can cause side effects in some animals.
  • Warfarin should not be used in animals with known hypersensitivity or allergy to the drug.
  • Warfarin should be avoided in animals with a history of bleeding or coagulation disorders.
  • Warfarin should be used with caution in animals undergoing major surgery.
  • Care must be taken if warfarin is given to animals with liver or kidney impairment, high blood pressure or during pregnancy (birth defects can occur).
  • Warfarin may interact with other medications. Consult with your veterinarian to determine if other drugs your pet is receiving could interact with warfarin. Such drugs include dimetidine, certain antibiotics, phenobarbital, steroid, sucralfate and spironolactone.
  • The most significant adverse effect of warfarin is bleeding because of difficulties in dosing this drug in animals. This bleeding can be quite serious and even fatal if not controlled.

    How Warfarin Is Supplied

  • Warfarin is available in 1 mg, 2 mg, 2.5 mg, 3 mg, 4 mg, 5 mg, 6 mg, 7.5 mg and 10 mg tablets.
  • It is also available in 2 mg vials for injection.

    Dosing Information

  • Medication should never be administered without first consulting your veterinarian.
  • In dogs, warfarin is dosed at 0.05 to 0.1 mg per pound (0.1 to 0.2 mg/kg) once daily.
  • In cats the usual initial dose is 0.5 mg per cat once daily.
  • Routine examinations and blood-clotting tests are required to determine proper and most effective doses.
  • The duration of administration depends on the condition being treated, response to the medication and the development of any adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian. Even if your pet feels better, the entire treatment plan should be completed to prevent relapse.

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