Warfarin

Warfarin is an oral vitamin K antagonist anticoagulant with a limited historical role in veterinary thrombosis management. It has been used for selected long-term prevention of recurrent thromboembolism, principally in dogs and cats. Its clinical effect develops as existing functional coagulation factors decline, making it different from immediately acting anticoagulants.

Group

Vitamin K antagonist anticoagulant

Mechanism of Action

Warfarin inhibits vitamin K epoxide reductase, limiting regeneration of reduced vitamin K. Reduced vitamin K is required for gamma-carboxylation of coagulation factors II, VII, IX and X and the regulatory proteins C and S. Newly synthesized proteins therefore have reduced functional activity. Anticoagulation develops as previously circulating functional clotting factors are cleared.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Anticoagulation — Initial	0.1–0.2 mg/kg	PO	Every 12 h	Up to 4 days; Initial 2-4 days in source; titrate with PT/INR.
Anticoagulation — Maintenance	0.1–0.2 mg/kg	PO	Every 24 h	Individualize by coagulation monitoring.
Rapid-onset fixed loading protocol — Loading	6 mg/animal	PO	Every 24 h	Up to 2 days; Two initial treatments; use PT/INR and patient response to individualize. This is distinct from the weight-based loading regimen.
Venous thrombosis or pulmonary thromboembolism — Initial	0.22 mg/kg	PO	Every 12 h	Titrate to PT; this report targets 1.25-1.5 times pretreatment, distinct from other protocol targets.
Proteinuric glomerular disease thromboprophylaxis — Initial	0.22 mg/kg	PO	Every 24 h	Adjust to monitored coagulation, not a fixed long-term dose.

Cats

Use / stage	Dose	Route	Schedule	Instructions
Anticoagulation — Initial	0.25–0.5 mg/animal	PO	Every 24 h	Adjust by PT/INR.
Anticoagulation — Initial	0.06–0.09 mg/kg	PO	Every 24 h	Alternative initial weight-based range.
Anticoagulation — Maintenance	0.1 mg/kg	PO	Every 24 h	Reported experience indicates PT prolongation; must individualize.
Venous thrombosis or pulmonary thromboembolism	0.5 mg/animal	PO	Every 24 h

Horses

Use / stage	Dose	Route	Schedule	Instructions
Reported anticoagulation treatment	0.02 mg/kg	PO	Every 24 h	Increase by 20% only after coagulation assessment; allow 7 days between dose changes. Reported goal is PT prolonged by 2-4 seconds.
Laminitis anticoagulant adjunct	0.0198 mg/kg	PO	Every 24 h	Historical use; monitor one-stage PT for 2-4-second prolongation.

Indications

Historical long-term anticoagulation for selected veterinary patients with venous thrombosis or pulmonary thromboembolism.
Prevention of recurrent thromboembolism in selected dogs or cats when warfarin is the chosen anticoagulant, with limited clinical outcome evidence.
Historical thromboprophylaxis in selected high-risk hypercoagulable patients, including some protein-losing glomerular disease cases, with unestablished mortality benefit.

Contraindications

Bleeding tendency
Other anticoagulant or antiplatelet therapy requires extreme care.

Interactions

Many drug and diet interactions: aspirin, phenylbutazone, azoles, cimetidine, chloramphenicol, sulfonamides, metronidazole and selected cephalosporins can increase bleeding.
Changing diet or concurrent enzyme-inducing/inhibiting medicines can alter anticoagulation; recheck PT after any relevant treatment change.

Adverse effects

Spontaneous internal or external bleeding, hemarthrosis and hemorrhage after injury.

Toxicity

Clinically significant anticoagulant bleeding requires vitamin K and appropriate supportive blood-product treatment. Hemorrhage may be delayed 2-5 days after exposure. Evaluate coagulation even before visible bleeding; clinically significant poisoning may require vitamin K1 and blood products.

Preparations

Preparation	Form	Strength	Route
Warfarin 1 mg	Tablet	1 mg / tablet	PO
Warfarin 2 mg	Tablet	2 mg / tablet	PO
Warfarin 2.5 mg	Tablet	2.5 mg / tablet	PO
Warfarin 4 mg	Tablet	4 mg / tablet	PO
Warfarin 5 mg	Tablet	5 mg / tablet	PO
Warfarin 7.5 mg	Tablet	7.5 mg / tablet	PO
Warfarin 10 mg	Tablet	10 mg / tablet	PO

Monitoring and Administration

Individualize using PT/INR, not a presumed universal amount. Reported targets PT 1.5-2 times control or INR 2-3; laboratory validation matters.

Tablet fractions may have uneven content; no equal-dose scoring is inferred. Protect from light.

Feline initiation usually overlaps a separately prescribed heparin course for approximately 4-7 days. Reported PT targets vary from 1.25-1.5 to twice baseline; INR may not be directly validated for cats. One monitoring plan checks daily during week 1, twice weekly for 2-4 weeks, then less often once stable. Reassess after concurrent medication changes. Equine daily 20% escalation conflicts with another recommendation to wait 7 days between changes and is not automated.