Section 13 · Monograph
Warfarin
Reviewed Nov 2026FDA label
Coumadin / JantovenVitamin K antagonist
- Forms
- Tab: 1, 2, 2.5, 3, 4, 5, 6, 7.5, 10mg
- Dose
- Individualized to INR goal (usually 2-3)
- MOA
- Inhibits vitamin K-dependent clotting factors II, VII, IX, X
Black Box Warning
- May cause major or fatal bleeding — perform regular INR monitoring; drugs, diet and other factors affect INR; instruct patients on bleeding risk
Indications
- Prophylaxis and treatment of venous thrombosis and pulmonary embolismFDA-labeled
- Thromboembolic complications of atrial fibrillation and cardiac valve replacementFDA-labeled
- Reduce risk of death, recurrent MI and thromboembolic events after MIFDA-labeled
Mechanism of Action
- Inhibits vitamin K epoxide reductase, reducing synthesis of vitamin K-dependent factors II, VII, IX, X and proteins C and S
Full Dosing
Adult / usual
- Typical start 2-5mg daily, adjusted to INR goal (usually 2-3; 2.5-3.5 for mechanical mitral valve)
- Lower starting dose in elderly, liver disease, malnutrition or with interacting drugs
- INR every few days at initiation, then up to every 4 weeks when stable
Contraindications
- Pregnancy (except mechanical valves in high-risk situations)
- Hemorrhagic tendencies, recent surgery with large open surfaces, threatened abortion, eclampsia
- Unsupervised patients with conditions limiting reliable monitoring
- Hypersensitivity
Drug Interactions
- CYP2C9/3A4/1A2 inhibitors (azoles, amiodarone, many antibiotics) raise INR
- Carbamazepine, phenytoin, phenobarbital, rifampin lower INR
- Changes in dietary vitamin K intake alter response
- NSAIDs and aspirin increase bleeding risk
Adverse Effects
- Bleeding
- Skin necrosis (early therapy; protein C deficiency)
- Purple toes syndrome
- Teratogenicity
Monitoring Parameters
- INR
- CBC
- Signs of bleeding
Pregnancy & Lactation
- Contraindicated in most pregnancies (embryopathy, fetal bleeding); LMWH preferred
- Compatible with breastfeeding