WARFARIN SODIUM
Clinical safety rating: avoid
A vast number of drugs can increase or decrease INR through various mechanisms Has a narrow therapeutic index and requires frequent INR monitoring to prevent bleeding or clotting.
Inhibits vitamin K epoxide reductase complex 1 (VKORC1), preventing reduction of vitamin K, thereby impairing activation of clotting factors II, VII, IX, X, and anticoagulant proteins C and S.
| Metabolism | Primarily hepatic via CYP2C9 (major), with contributions from CYP3A4, CYP1A2, CYP2C19, and CYP2C8. Metabolites are excreted in urine and feces. |
| Excretion | Primarily renal as inactive metabolites (92%), with minimal biliary/fecal (8%). The S-enantiomer is metabolized by CYP2C9 to inactive metabolites; the R-enantiomer is metabolized by CYP1A2 and CYP3A4. Less than 2% is excreted unchanged in urine. |
| Half-life | Terminal elimination half-life is approximately 20-60 hours (mean 40 hours) for the S-enantiomer and 37-89 hours for the R-enantiomer. The clinical context: anticoagulant effect persists for 2-5 days due to the half-life of clotting factors (II, VII, IX, X), with factor VII (half-life ~6h) being the first affected; full effect requires 3-5 days to achieve steady-state. |
| Protein binding | Primarily bound to albumin (97-99%). The free fraction is the pharmacologically active moiety. |
| Volume of Distribution | Approximately 0.14 L/kg. This low Vd indicates limited extravascular distribution, consistent with high protein binding and confinement to the vascular space, primarily binding to albumin. |
| Bioavailability | Oral: >95% (immediate-release tablets). Warfarin has complete absorption from the gastrointestinal tract. Food can delay but does not significantly reduce absorption. |
| Onset of Action | Oral: Anticoagulant effect (measured by INR) typically begins within 24-72 hours; maximal effect occurs at 72-96 hours. Intravenous: Similar to oral; not typically administered IV. |
| Duration of Action | Duration of anticoagulant effect persists for 2-5 days after cessation, depending on dose and patient factors. Clinical note: Reversal may require vitamin K (oral or IV) or fresh frozen plasma/prothrombin complex concentrate for urgent reversal. |
2.5-10 mg orally once daily, adjusted based on INR; typical maintenance dose 2-7.5 mg/day.
| Dosage form | TABLET |
| Renal impairment | No dose adjustment required based on GFR; warfarin is highly protein bound and not renally cleared. Monitor INR closely in renal impairment due to altered protein binding. |
| Liver impairment | Child-Pugh A: no adjustment; Child-Pugh B: reduce dose by 25-50% and monitor INR closely; Child-Pugh C: avoid use or use extreme caution with dose reduction >50%. |
| Pediatric use | Initial dose 0.1-0.2 mg/kg orally once daily (max 5 mg); adjust based on INR; typical maintenance 0.05-0.34 mg/kg/day. |
| Geriatric use | Start with lower initial dose (2-3 mg/day) due to increased sensitivity; adjust cautiously; frequent INR monitoring recommended. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
A vast number of drugs can increase or decrease INR through various mechanisms Has a narrow therapeutic index and requires frequent INR monitoring to prevent bleeding or clotting.
| FDA category | Positive |
| Breastfeeding | Warfarin is excreted into breast milk in negligible amounts. The M/P ratio is approximately 0.2. It is considered compatible with breastfeeding, but monitor infant for bruising or bleeding. |
| Teratogenic Risk | Warfarin sodium is teratogenic. First trimester exposure is associated with warfarin embryopathy (nasal hypoplasia, stippled epiphyses) in 5-30% of exposed fetuses. Second and third trimester exposure can cause CNS abnormalities, optic atrophy, microcephaly, mental retardation, and fetal hemorrhage. Risk is highest between weeks 6 and 12 of gestation. |
■ FDA Black Box Warning
Warfarin can cause major or fatal bleeding. Risk factors include INR >4, age, comorbidities, and drug interactions. Patients should be monitored regularly for INR. Use cautiously with conditions that increase bleeding risk.
| Common Effects | Bleeding |
| Serious Effects |
Hypersensitivity to warfarin or any component; hemorrhagic tendencies (e.g., hemophilia, thrombocytopenia); recent or potential surgery; active bleeding; severe uncontrolled hypertension; pregnancy; major regional or lumbar block anesthesia; unreliable patient with inability to monitor INR.
| Precautions | Risk of hemorrhage; monitor INR frequently. Necrosis or gangrene of skin or other tissues due to protein C deficiency. Systemic atheroemboli (purple toes syndrome). Acute kidney injury (warfarin-related nephropathy). Pregnancy: crosses placenta, causing fetal hemorrhage and spontaneous abortion. |
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| Fetal Monitoring | Monitor maternal INR weekly or more frequently as needed. Fetal monitoring includes serial ultrasound for growth and anomalies, and fetal echocardiography due to risk of CNS defects. Assess for signs of fetal hemorrhage (e.g., fetal distress). |
| Fertility Effects | Warfarin does not appear to adversely affect fertility. However, due to its teratogenicity, women of childbearing potential should use effective contraception during therapy. |