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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

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

Frequency

q24h

🐈

Cat

Used for ATE prevention (controversial — LMWHs and clopidogrel more commonly use

Dose

0.06–0.1 mg/kg

Route

Frequency

q24h

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 →

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

Educational reference only

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.

Educational reference only

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.

Educational reference only

References

The PetCare.AI drug reference is built from 13 authoritative sources cited across 580 drug monographs.

Textbooks & handbooks — 5

Plumb's Veterinary Drug Handbook
Withrow & MacEwen's Small Animal Clinical Oncology
Merck Veterinary Manual
NRC Nutrient Requirements of Dogs and Cats
Veterinary Pharmacology and Therapeutics (Riviere & Papich)

Clinical guidelines & consensus — 4

ASPCA Animal Poison Control Center Guidelines
AAHA Diabetes Management Guidelines
ASPCA Poison Control Guidelines
RECOVER CPR Guidelines

Journals & peer-reviewed studies — 2

EPIC Study (J Vet Intern Med 2016)
JVIM FAT CAT Study

Regulatory & approvals — 1

CDSCO Veterinary Drug Approval Registry (1969–2026)

Databases — 1

Washington State University VCPL MDR1 Database