Warfarin Sodium
**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: 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 ฮณ-carboxylation (activation) of **coagulation factors II, VII, IX, and X**, as well as the endogenous anticoagulant proteins C and S. * **Pathway:** VKORC1 inhibition โ Depletion of active Vitamin K โ Production of inactive, undercarboxylated clotting factors (PIVKA) โ Prolonged clotting times and anticoagulation.
Dosing by species
- Adjunctive maintenance therapy for venous thrombosis with or without pulmonary thromboembolism (PTE) ยท 0.5 mg (total dose per cat) ยท PO ยท once daily ยท Initially, then adjusted to achieve a PT prolongation of 1.25 to 1.5 times the pretreatment value.
- Maintenance therapy for feline arterial thromboembolism ยท 0.06 to 0.09 mg/kg/day ยท PO ยท once daily ยท Heparinization is recommended during the first 5 to 7 days that warfarin is administered.
- Long-term thromboprophylaxis ยท 0.06-0.09 mg/kg per day ยท PO ยท once daily ยท 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 ยท 0.25-0.5 mg (total dose) per cat ยท PO ยท once daily ยท Initially. Adjust dosage to prolong PT to twice normal value, or INR to be between 2-3. Overlap therapy with heparin.
- Cardiogenic arterial thromboembolism ยท 0.25-0.5 mg total dose per cat ยท PO ยท q24h ยท 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.
- Adjunctive treatment of laminitis ยท 0.0198 mg/kg ยท PO ยท once daily ยท Monitor OSPT (one-step prothrombin time) until prolonged 2-4 seconds beyond baseline.
- Anticoagulant ยท 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. ยท PO ยท once daily ยท ARCI UCGFS Class 5 Drug.
Routes of administration
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
Drug interactions
- Acetaminophen, NSAIDs, Salicylates ยท May increase anticoagulant response (via protein displacement or platelet inhibition); increases bleeding risk.
- Fluoroquinolones, Macrolides (Azithromycin, Erythromycin), Chloramphenicol, Sulfonamides, Co-trimoxazole ยท May increase anticoagulant response (often via inhibition of hepatic metabolism).
- Cimetidine, Cisapride, Fluoxetine, Metronidazole, Sertraline ยท May increase anticoagulant response.
- Amiodarone, Allopurinol, Danazol, Diazoxide, Ethacrynic acid, Pentoxifylline, Propylthiouracil, Quinidine, Zafirlukast ยท May increase anticoagulant response.
- Anabolic Steroids, Thyroid Medications ยท May increase anticoagulant response.
- Heparin ยท Increases anticoagulant response; often used concurrently during the initial overlap phase but requires careful monitoring.
- Barbiturates (e.g., Phenobarbital), Rifampin, Griseofulvin ยท May decrease anticoagulant response (via induction of hepatic metabolizing enzymes).
- Corticosteroids, Estrogens, Mercaptopurine, Spironolactone, Sucralfate ยท May decrease anticoagulant response.
- Vitamin K ยท 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)
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)**.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.