COUMADIN

Clinical safety rating: caution

Comprehensive clinical and safety monograph for COUMADIN (COUMADIN).

How it works

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.

What the body does with it

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.

Classification & Brands

Dosing & administration

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.

Use during pregnancy

1st trimester	Consult provider
2nd trimester	Consult provider
3rd trimester	Consult provider

Clinical note

Comprehensive clinical and safety monograph for COUMADIN (COUMADIN).

Breastfeeding	Warfarin is excreted into breast milk in negligible amounts; M/P ratio approximately 0.2. Considered compatible with breastfeeding by AAP. Monitor infant for signs of bleeding.
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

Warnings & precautions

■ 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.

Side Effect Profile

Serious Effects

Absolute Contraindications

["Pregnancy (Category X)","Hemorrhagic tendencies or blood dyscrasias","Recent or contemplated surgery of the central nervous system or eye","Threatened abortion","Unsupervised patients with a lack of appropriate laboratory monitoring","Major regional or lumbar block anesthesia","Malignant hypertension","Known hypersensitivity to warfarin or any component of the product"]

Clinical Precautions

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)"]

Clinical Tips & Counseling

Drug interactions

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COUMADIN

Clinical safety rating: caution

Comprehensive clinical and safety monograph for COUMADIN (COUMADIN).

How it works

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.

What the body does with it

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.

Classification & Brands

Dosing & administration

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.

Use during pregnancy

1st trimester	Consult provider
2nd trimester	Consult provider
3rd trimester	Consult provider

Clinical note

Comprehensive clinical and safety monograph for COUMADIN (COUMADIN).

Breastfeeding	Warfarin is excreted into breast milk in negligible amounts; M/P ratio approximately 0.2. Considered compatible with breastfeeding by AAP. Monitor infant for signs of bleeding.
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