Warfarin Sodium
Also known as: Coumadin · Jantoven · Panwarfin · sodium warfarin · warfarinum natricum
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Adjunctive maintenance therapy for venous thrombosis with or without pulmonary thromboembolism (PTE) | 0.22 mg/kg | PO | q12h | — | 🌎 NA |
| Adjunctive therapy of thromboemboli | 0.22 mg/kg | PO | q12h | — | 🌎 NA |
| Prophylactic use in patients with glomerular disease and severe proteinuria | 0.22 mg/kg | PO | once daily | — | 🌎 NA |
- Adjunctive maintenance therapy for venous thrombosis with or without pulmonary thromboembolism (PTE): Initially, then adjusted to achieve a PT prolongation of 1.25 to 1.5 times the pretreatment value.
- Adjunctive therapy of thromboemboli: Target dosage to prolong PT by 1.25-1.5 times the pretreatment value.
- Prophylactic use in patients with glomerular disease and severe proteinuria: Initially. Monitor PT and adjust dose so that PT is maintained at 1.5 times normal.
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Adjunctive maintenance therapy for venous thrombosis with or without pulmonary thromboembolism (PTE) | 0.5 mg (total dose per cat) | PO | once daily | — | 🌎 NA |
| Maintenance therapy for feline arterial thromboembolism | 0.06 to 0.09 mg/kg/day | PO | once daily | — | 🌎 NA |
| Long-term thromboprophylaxis | 0.06-0.09 mg/kg per day | PO | once daily | — | 🌎 NA |
| Adjunctive therapy of thromboembolism | 0.25-0.5 mg (total dose) per cat | PO | once daily | — | 🌎 NA |
| Cardiogenic arterial thromboembolism | 0.25-0.5 mg total dose per cat | PO | q24h | — | 🌎 NA |
- Adjunctive maintenance therapy for venous thrombosis with or without pulmonary thromboembolism (PTE): Initially, then adjusted to achieve a PT prolongation of 1.25 to 1.5 times the pretreatment value.
- Maintenance therapy for feline arterial thromboembolism: Heparinization is recommended during the first 5 to 7 days that warfarin is administered.
- Long-term thromboprophylaxis: Initially. Tablets should be crushed and mixed well due to unequal drug distribution. Overlap heparin and warfarin therapy by at least 4-5 days.
- Adjunctive therapy of thromboembolism: Initially. Adjust dosage to prolong PT to twice normal value, or INR to be between 2-3. Overlap therapy with heparin.
- Cardiogenic arterial thromboembolism: Heparin (150-250 Units/kg SC q8h) should be administered concurrently during the first 4-6 days. Target PT range is 1.3-1.6 times baseline, or INR 2.0-3.0.
Horses
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Adjunctive treatment of laminitis | 0.0198 mg/kg | PO | once daily | — | 🌎 NA |
| Anticoagulant | Initially, 0.018 mg/kg PO once daily and increase dose by 20% every day until baseline PT is doubled. Final dose rates may be from 0.012 mg/kg to 0.57 mg/kg daily. | PO | once daily | — | 🌎 NA |
- Adjunctive treatment of laminitis: Monitor OSPT (one-step prothrombin time) until prolonged 2-4 seconds beyond baseline.
- Anticoagulant: ARCI UCGFS Class 5 Drug.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Warfarin is a coumarin-derivative anticoagulant used primarily for the long-term treatment or prevention of thrombotic conditions in veterinary patients, such as cats, dogs, and horses.
Due to its narrow therapeutic index, the potential for life-threatening hemorrhage, and the need for frequent and costly coagulation monitoring, its use in veterinary medicine is somewhat controversial and often reserved for specific cases where other anticoagulants are not feasible.
Clinical Pearls:
- Warfarin is administered as a racemic mixture of S(+) and R(-) enantiomers, with the S-enantiomer being significantly more potent.
- Because it only inhibits the synthesis of new clotting factors, there is a characteristic lag time of 2-5 days before full anticoagulant effects are observed, as pre-existing circulating factors must first degrade.
- Warfarin is the active ingredient in many first-generation anticoagulant rodenticides; toxicity presents identically to clinical overdose.
Mechanism of action
Warfarin acts as an indirect anticoagulant by antagonizing Vitamin K.
- It competitively inhibits the enzyme Vitamin K epoxide reductase (VKORC1).
- This inhibition prevents the conversion of inactive Vitamin K epoxide back into its active form (Vitamin K hydroquinone).
- Active Vitamin K is an essential cofactor for the γ-carboxylation (activation) of coagulation factors II, VII, IX, and X, as well as the endogenous anticoagulant proteins C and S.
- Pathway: VKORC1 inhibition → Depletion of active Vitamin K → Production of inactive, undercarboxylated clotting factors (PIVKA) → Prolonged clotting times and anticoagulation.
Safety & warnings
Contraindications
- Preexistent hemorrhagic tendencies or diseases
- Patients undergoing or contemplating eye or CNS surgery
- Major regional lumbar block anesthesia
- Surgery of large, open surfaces
- Active bleeding from the GI, respiratory, or GU tract
- Aneurysm
- Acute nephritis
- Cerebrovascular hemorrhage
- Blood dyscrasias
- Uncontrolled or malignant hypertension
- Hepatic insufficiency
- Pericardial effusion
- Pregnancy (Teratogenic - FDA Category X / Papich Class D)
- Visceral carcinomas
Adverse effects
- Dose-related hemorrhage (primary adverse effect)
- Anemia
- Thrombocytopenia
- Weakness
- Hematomas and ecchymoses
- Epistaxis (nosebleeds)
- Hematemesis (vomiting blood)
- Hematuria (blood in urine)
- Melena (dark, tarry stools)
- Hematochezia (fresh blood in stools)
- Hemarthrosis (bleeding into joints)
- Hemothorax
- Intracranial hemorrhage
- Pericardial hemorrhage
- Death
Precautions
Strict Monitoring Required: Warfarin has a narrow therapeutic index. Frequent monitoring of coagulation status (PT/INR) is mandatory to prevent fatal hemorrhage.
- Teratogenicity: Warfarin is embryotoxic and causes congenital malformations. It is strictly contraindicated in pregnancy. Low-dose heparin is preferred if anticoagulation is required during pregnancy.
- Nursing: Use with caution in nursing patients; prolonged prothrombin times have been reported in some breast-fed human infants.
- Dietary Changes: Sudden changes in diet (especially foods high in Vitamin K) can alter the efficacy of warfarin.
- Laboratory Interference: May cause falsely decreased theophylline values if using the Schack and Waxler ultraviolet method.
Drug interactions
May increase anticoagulant response (via protein displacement or platelet inhibition); increases bleeding risk.
May increase anticoagulant response (often via inhibition of hepatic metabolism).
May increase anticoagulant response.
May increase anticoagulant response.
May increase anticoagulant response.
Increases anticoagulant response; often used concurrently during the initial overlap phase but requires careful monitoring.
May decrease anticoagulant response (via induction of hepatic metabolizing enzymes).
May decrease anticoagulant response.
Directly antagonizes warfarin, decreasing its anticoagulant response (used as an antidote for overdose).
Monitoring
- Prothrombin Time (PT) - Target usually 1.25 to 1.5x (or up to 2x) normal depending on protocol
- International Normalized Ratio (INR) - Target usually 2.0 to 3.0 (Note: not fully validated for veterinary patients)
- PIVKA (Proteins Induced by Vitamin K Antagonists) - May be more sensitive than PT
- Platelet counts and hematocrit (PCV)
- Occult blood in stool and urine
- Physical observations for bleeding (mucous membranes, bruising, epistaxis)
- Clinical efficacy (resolution/prevention of thromboembolism)
Pharmacokinetics
Half-life
Absorption
Rapidly and completely absorbed after oral administration in humans and cats.
Distribution
Highly bound to plasma proteins (approximately 99% in humans, >96% in cats). Wide species variation exists; horses have a higher free (unbound) fraction than rats, sheep, or swine. Only the free fraction is active. Does not enter milk in humans.
Metabolism
Principally metabolized in the liver to inactive metabolites.
Elimination
Metabolites are excreted in urine and bile (then reabsorbed and excreted in the urine).
Overdose
Acute overdosages of warfarin may result in life-threatening hemorrhage.
- Toxic Doses: In dogs and cats, single doses of 5-50 mg/kg have been associated with toxicity. Cumulative toxic doses are reported as 1-5 mg/kg for 5-15 days in dogs, and 1 mg/kg for 7 days in cats.
- Lag Time: A lag time of 2-5 days may occur before clinical signs of toxicity manifest. Animals must be monitored closely during this period.
- Treatment: If detected early, prevent absorption from the gut using standard decontamination protocols (emesis, activated charcoal). If clinical signs of bleeding are noted, treat with blood products (plasma/whole blood to replace active clotting factors) and Vitamin K1 (phytonadione).
Available products
Formulations
- Oral tablets
- Powder for injection (lyophilized)
Veterinary
- None (ARCI Class 5 substance for racing)
Human-labeled
- Warfarin Sodium Oral Tablets (scored): 1 mg, 2 mg, 2.5 mg, 3 mg, 4 mg, 5 mg, 6 mg, 7.5 mg & 10 mg (Coumadin, Jantoven, generic)
- Warfarin Sodium Powder for Injection, lyophilized: 5.4 mg (2 mg/mL when reconstituted) preservative-free in 5 mg vials (Coumadin)
Regulatory status
No regulatory data for: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Tablets should be stored in tight, light-resistant containers at temperatures less than 40°C, preferably at room temperature. Powder for injection should be protected from light and used immediately after reconstituting.
Client information
Critical Warning: Warfarin is a potent blood thinner. Strict adherence to the prescribed dosing schedule is absolutely essential. Do not skip doses or give extra doses without consulting your veterinarian.
- Watch for Bleeding: Immediately report any signs of bleeding to your veterinarian. This includes pale gums, unusual bruising, nosebleeds, blood in the urine or stool (which may look black and tarry), vomiting blood, coughing up blood, or sudden weakness/lethargy.
- Prevent Trauma: Keep pets (especially cats) indoors and restrict rough play to minimize the risk of traumatic injury, which could lead to severe bleeding.
- Medication Interactions: Do not give your pet any new medications, supplements, or over-the-counter drugs (especially aspirin or other pain relievers) without checking with your veterinarian, as many drugs interact dangerously with warfarin.
- Diet: Keep your pet's diet consistent, as sudden changes in food (especially those containing Vitamin K) can alter how the drug works.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.