COUMADIN
Clinical safety rating: caution
Comprehensive clinical and safety monograph for COUMADIN (COUMADIN).
Inhibits vitamin K epoxide reductase complex 1 (VKORC1), thereby decreasing the synthesis of vitamin K-dependent clotting factors II, VII, IX, and X, as well as anticoagulant proteins C and S.
| Metabolism | Hepatic metabolism primarily via CYP2C9 (S-enantiomer) and to a lesser extent CYP1A2, CYP3A4 (R-enantiomer). Metabolites are excreted in urine and feces. |
| Excretion | Renal (approximately 92% as inactive metabolites), fecal/biliary (minor, approximately 8%). Less than 2% excreted unchanged. |
| Half-life | Terminal elimination half-life: 20–60 hours (mean ~40 hours); clinically, anticoagulant effect persists for 2–5 days after stopping due to hepatic synthesis of functional clotting factors. |
| Protein binding | 97–99% bound, primarily to albumin. |
| Volume of Distribution | 0.11–0.2 L/kg; low Vd indicates limited extravascular distribution, consistent with high plasma protein binding. |
| Bioavailability | Oral: >90% (well absorbed); available as tablets (1, 2, 2.5, 3, 4, 5, 6, 7.5, 10 mg). |
| Onset of Action | Oral: Anticoagulant effect (INR elevation) begins 24–72 hours; full therapeutic effect in 3–5 days. No parenteral formulation. |
| Duration of Action | 2–5 days after last dose, depending on dose and patient factors; effect prolonged in liver disease or vitamin K deficiency. |
| Molecular Weight | 308.33 |
Initial dose 2-5 mg orally once daily, adjusted based on INR response; typical maintenance dose 2-10 mg/day.
| Dosage form | TABLET |
| Renal impairment | No specific GFR-based dose adjustment; monitor INR closely in renal impairment due to increased bleeding risk. |
| Liver impairment | Child-Pugh Class A: no adjustment; Child-Pugh B: reduce initial dose and titrate cautiously; Child-Pugh C: contraindicated or use extreme caution due to impaired clotting factor synthesis. |
| Pediatric use | Initial dose 0.1-0.2 mg/kg/day orally once daily, max 10 mg/day; adjust based on INR. |
| Geriatric use | Start at lower end of dosing range (2-3 mg/day) due to increased sensitivity; monitor INR more frequently. |
| 1st trimester | Contraindicated: warfarin is teratogenic (warfarin embryopathy) with fetal abnormalities including nasal hypoplasia and stippled epiphyses. Use is not recommended; alternative anticoagulation is preferred. |
| 2nd trimester | Use with caution: risk of embryopathy is lower, but may cause CNS abnormalities. Warfarin crosses the placenta and can cause fetal hemorrhage. Risk-benefit assessment required, consider low-dose regimens (e.g., target INR 2.0-2.5). |
| 3rd trimester | Contraindicated: high risk of fetal hemorrhage and intracranial bleeding during labor. Discontinue and switch to heparin before delivery (usually at 36-37 weeks). |
Clinical note
Comprehensive clinical and safety monograph for COUMADIN (COUMADIN).
| Placental transfer | Yes: warfarin readily crosses the placenta (approximately 50-60% of maternal concentration in fetal plasma). It is a low molecular weight compound (MW 308.33) that can achieve equilibrium across the placenta. |
| Breastfeeding |
■ FDA Black Box Warning
Warfarin can cause major or fatal bleeding. Bleeding is more likely to occur during the starting period and with higher doses. Monitor patients regularly for signs and symptoms of bleeding. Advise patients to report any signs or symptoms of bleeding. Concomitant use of warfarin with NSAIDs, aspirin, or other drugs affecting hemostasis increases the risk of bleeding.
| Serious Effects |
Pregnancy (especially first and third trimesters)Active bleeding or hemorrhagic tendencies (e.g., hemophilia, thrombocytopenia)Recent or impending surgery of the central nervous system or eyeSevere uncontrolled hypertensionMalignant hypertensionBacterial endocarditisThreatened abortionNeuraxial anesthesia (e.g., spinal or epidural) due to risk of spinal hematoma
| Precautions | Risk of major or fatal bleeding, Necrosis of skin and other tissues, Systemic atheroemboli and cholesterol microemboli, Use in patients with protein C or S deficiency increases risk of skin necrosis, Heparin-induced thrombocytopenia and thrombosis, Pregnancy: Category X (can cause fetal harm; use contraindicated) |
| Food/Dietary |
Loading safety data…
| Warfarin is excreted into breast milk in very small amounts, insufficient to cause anticoagulant effects in the infant. The American Academy of Pediatrics considers warfarin compatible with breastfeeding. Monitor infant for signs of bleeding if high maternal doses are used. |
| Lactation Rating | L3 (Moderately Safe) - limited data, but appears safe in usual doses |
| Teratogenic Risk | Pregnancy Category X. Warfarin causes fetal abnormalities in up to 30% of first trimester exposures (nasal hypoplasia, stippled epiphyses, CNS defects). Second and third trimester use risks fetal hemorrhage, CNS abnormalities, and spontaneous abortion. Avoid entirely in pregnancy; use heparin-based therapy. |
| Fetal Monitoring | Monitor maternal INR weekly; adjust dose to maintain therapeutic INR (typically 2-3). Fetal surveillance: serial ultrasound for anomalies, growth, and evidence of hemorrhage. Consider fetal echocardiogram if first trimester exposure. |
| Fertility Effects | No direct impairment of fertility reported. However, warfarin's anticoagulant effect may increase risk of hemorrhagic corpus luteum or other gynecologic bleeding complications during ovulation and early pregnancy. |
| Avoid large changes in intake of vitamin K-rich foods (e.g., broccoli, Brussels sprouts, cabbage, kale, spinach, collard greens, Swiss chard, parsley). Consistent intake is key; do not suddenly increase or decrease these foods. Limit alcohol (max 1-2 drinks/day) as it can affect INR. Cranberry juice, grapefruit juice, and pomegranate juice may increase INR and risk of bleeding. Herbal supplements such as St. John's Wort, ginkgo, ginseng, and feverfew can interact; avoid without medical advice. |
| Clinical Pearls | Initiate warfarin at 5 mg daily for most patients, but adjust for elderly, low body weight, or hepatic impairment. Monitor INR closely; goal INR 2-3 for most indications. Avoid loading doses. Use bridging with heparin for high-thrombotic-risk patients during initiation. Consider pharmacogenetic testing for CYP2C9 and VKORC1 variants. Avoid IM injections due to hematoma risk. Reversal: vitamin K for mild elevation, FFP or PCC for serious bleeding with elevated INR. |
| Patient Advice | Take warfarin exactly as prescribed at the same time each day. · Regular blood tests (INR) are required to ensure correct dose and prevent bleeding or clotting. · Report any unusual bleeding, bruising, dark stools, or red urine immediately. · Avoid starting or stopping any new medications, including over-the-counter drugs and supplements, without consulting your doctor. · Use a soft toothbrush and electric razor to minimize bleeding; avoid activities with risk of injury. · Carry a medical alert card or wear a bracelet indicating you take warfarin. · Do not change your diet suddenly, especially foods high in vitamin K (e.g., spinach, kale, broccoli). |