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Warfarin

Prescription
Vitamin K Antagonist Anticoagulant
Last reviewed 21 Apr 2026 · PetCare.AI Editorial Team
Species
Dog, Cat
Brands
2 available
Interactions
8 documented
Formulations
1

Mechanism of action

Inhibits vitamin K epoxide reductase (VKOR), blocking recycling of vitamin K and preventing gamma-carboxylation of clotting factors II, VII, IX, and X. Full anticoagulant effect delayed 3–5 days (existing factors must be cleared). Narrow therapeutic index.

At a glance

Class
Vitamin K Antagonist Anticoagulant
Schedule
Prescription
Storage
Store at room temperature, protect from light and moisture

Dosing

🐕

Dog

Used for chronic thromboembolic disease prevention (IMHA maintenance, cardiac di
Dose
0.1–0.2 mg/kg
Route
PO
Frequency
q24h
🐈

Cat

Used for ATE prevention (controversial — LMWHs and clopidogrel more commonly use
Dose
0.06–0.1 mg/kg
Route
PO
Frequency
q24h

Formulations

💊

Other — 1

Strength
—

Storage

Store at room temperature, protect from light and moisture

Safety

Monitoring parameters

PT/INR q2–3 days until stable, then weekly, then biweeklySigns of bleeding (petechiae, melena, hematuria, epistaxis)Drug interaction review at every visitDietary vitamin K consistency

Interactions

Major — 5

Meloxicam
major
NSAIDs inhibit platelet function (COX-1 dependent TXA2) and displace warfarin from protein binding, greatly increasing hemorrhage risk.
Management: Avoid concurrent use. If NSAID essential, monitor PT/INR every 2-3 days. Use gastroprotectant. Watch for melena, petechiae.
Metronidazole
major
Metronidazole inhibits CYP2C9 metabolism of warfarin's more active S-enantiomer, increasing warfarin effect and hemorrhage risk.
Management: Reduce warfarin dose by 25-50% when adding metronidazole. Monitor PT/INR every 2-3 days during metronidazole course.
Fluconazole
major
Fluconazole inhibits CYP2C9 and CYP3A4, significantly increasing warfarin levels and hemorrhage risk.
Management: Reduce warfarin dose by 25-50%. Monitor PT/INR every 2-3 days. Consider alternative antifungal.
Phytonadione (Vitamin K1)
major
Vitamin K1 directly antagonizes warfarin's mechanism of action (restores vitamin K-dependent clotting factor carboxylation). Used therapeutically as warfarin antidote, but also interferes with warfarin therapy.
Management: Intentional use as warfarin antidote for over-anticoagulation. If patient needs both anticoagulation and has dietary Vitamin K variation, maintain consistent Vitamin K intake.
Bismuth Subsalicylate
major
Salicylate component of bismuth subsalicylate inhibits platelet function and may displace warfarin from protein binding, greatly increasing hemorrhage risk.
Management: Avoid in patients on warfarin. Use non-salicylate GI protectant (omeprazole, sucralfate) instead.

Moderate — 3

Phenobarbital
moderate
Phenobarbital induces CYP enzymes metabolizing warfarin, reducing anticoagulant effect.
Management: May need warfarin dose increase during phenobarbital therapy. Monitor PT/INR closely when adding or discontinuing phenobarbital.
Griseofulvin
moderate
Griseofulvin induces CYP enzymes metabolizing warfarin, reducing anticoagulant effect.
Management: Monitor PT/INR. May need warfarin dose increase during griseofulvin course.
Pentoxifylline
moderate
Pentoxifylline inhibits platelet aggregation and may increase warfarin effect. Additive bleeding risk.
Management: Monitor PT/INR more frequently. Watch for bleeding signs.

Brands

Other markets

Coumadin
Jantoven

FAQs

Frequently asked questions

›What is Warfarin?
Warfarin is a vitamin k antagonist anticoagulant used in pets. Inhibits vitamin K epoxide reductase (VKOR), blocking recycling of vitamin K and preventing gamma-carboxylation of clotting factors II, VII, IX, and X. Full anticoagulant effect delayed 3–5 days (existing factors must be cleared). Narrow therapeutic index.
›What is Warfarin used for in pets?
Warfarin is used in veterinary medicine for: Used for chronic thromboembolic disease prevention (IMHA maintenance, cardiac di; Used for ATE prevention (controversial — LMWHs and clopidogrel more commonly use.
›What is the Warfarin dose for dogs?
For dogs, Warfarin is typically dosed as follows — Used for chronic thromboembolic disease prevention (IMHA maintenance, cardiac di: 0.1–0.2 mg/kg PO q24h. Always consult your veterinarian for a dose tailored to your pet's weight, age, and condition.
›What is the Warfarin dose for cats?
For cats, Warfarin is typically dosed as follows — Used for ATE prevention (controversial — LMWHs and clopidogrel more commonly use: 0.06–0.1 mg/kg PO q24h. Always consult your veterinarian for a dose tailored to your pet's weight, age, and condition.
›Does Warfarin need a prescription?
Yes. Warfarin is a prescription medication and should only be administered under veterinary supervision.

References

References

Textbooks & handbooks

  • Plumb, D.C. Plumb's Veterinary Drug Handbook. 10th ed., Wiley-Blackwell, 2023.
  • Vail, D.M., Thamm, D.H., & Liptak, J.M. (eds.). Withrow & MacEwen's Small Animal Clinical Oncology. 6th ed., Saunders/Elsevier, 2020.
  • Riviere, J.E., & Papich, M.G. (eds.). Veterinary Pharmacology and Therapeutics. 10th ed., Wiley-Blackwell, 2018.
  • National Research Council. Nutrient Requirements of Dogs and Cats. National Academies Press, Washington DC, 2006.
  • The Merck Veterinary Manual. Merck & Co., Online edition. https://www.merckvetmanual.com/

Clinical guidelines & consensus

  • Fletcher, D.J., Boller, M., Brainard, B.M., et al. "RECOVER Evidence and Knowledge Gap Analysis on Veterinary CPR." Journal of Veterinary Emergency and Critical Care, 2012;22(S1):S102–S131.
  • American Animal Hospital Association. 2018 AAHA Diabetes Management Guidelines for Dogs and Cats. AAHA Press.

Journals & peer-reviewed studies

  • Hogan, D.F., Fox, P.R., Jacob, K., et al. "Secondary prevention of cardiogenic arterial thromboembolism in the cat: The FAT CAT study." Journal of Veterinary Cardiology, 2015;17(Suppl 1):S306–S317.
  • Boswood, A., Häggström, J., Gordon, S.G., et al. "Effect of Pimobendan in Dogs with Preclinical Myxomatous Mitral Valve Disease and Cardiomegaly: The EPIC Study — A Randomized Clinical Trial." Journal of Veterinary Internal Medicine, 2016;30(6):1765–1779.
  • ASPCA Animal Poison Control Center. Toxicology and Poison Management Guidelines. American Society for the Prevention of Cruelty to Animals. https://www.aspca.org/pet-care/animal-poison-control

Regulatory & approvals

  • Central Drugs Standard Control Organisation (CDSCO), Government of India. Veterinary Drug Approval Registry, 1969–2026. Directorate General of Health Services. https://cdsco.gov.in/

Databases

  • Washington State University, College of Veterinary Medicine. Veterinary Clinical Pharmacology Laboratory (VCPL) — MDR1 Multidrug Sensitivity Database. https://vcpl.vetmed.wsu.edu/
Educational reference only
This information is provided for educational purposes and is not a substitute for professional veterinary advice, diagnosis, or treatment. Always consult a qualified veterinarian before administering any medication to your pet. Find a vet near you →