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. |
| Molecular Weight | 330.31 |
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 | Teratogenic: causes fetal warfarin syndrome (nasal hypoplasia, stippled epiphyses) in 5-30% of exposed pregnancies. Use contraindicated; consider heparin alternatives. |
| 2nd trimester | Fetal hemorrhage and central nervous system abnormalities possible. Avoid use; higher risk of intracranial bleeding. |
| 3rd trimester | Risk of fetal hemorrhage, especially during delivery; may cause neonatal anticoagulation. Avoid near term; switch to heparin. |
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 |
| Placental transfer | Crosses placenta; fetal levels ~50% of maternal serum concentrations due to low molecular weight and lipophilicity. Avoid use in pregnancy. |
■ 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 |
Active bleeding or hemorrhagic tendenciesRecent or impending surgery with major bleeding risk (including ophthalmic surgery)Recent or impending lumbar puncture, spinal anesthesia, or epidural anesthesiaSevere uncontrolled hypertensionPericarditis or pericardial effusionHistory of warfarin-induced skin necrosisPregnancy (especially first and third trimesters)Lactation (if infant has vitamin K deficiency or other coagulopathy)Hypersensitivity to warfarin or excipients
| 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. |
Loading safety data…
| Breastfeeding | Warfarin sodium does not pass into breast milk in clinically significant amounts due to high protein binding and high molecular weight. Considered compatible with breastfeeding; monitor infant for bruising or bleeding. |
| Lactation Rating | L2 (Safer) – limited data but no increased risk in infants. |
| 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. |
| 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. |
| Food/Dietary | Avoid or maintain consistent intake of foods high in vitamin K: liver, leafy greens (e.g., spinach, kale, Swiss chard, collard greens, turnip greens, beet greens), broccoli, Brussels sprouts, cabbage, asparagus, green tea, soybean oil, canola oil, and certain legumes. Avoid large amounts of cranberry juice, grapefruit juice, and pomegranate juice. Limit alcohol consumption. Vitamin K supplements should only be taken under medical supervision. |
| Clinical Pearls | Initiate warfarin at 5 mg/day for most patients, adjusting based on INR. Avoid loading doses. Monitor INR at least weekly during initiation. CYP2C9 and VKORC1 genotyping can guide dosing. Administer at same time daily, preferably in evening. For urgent reversal, use 4-factor PCC (Kcentra) or FFP. Vitamin K 1-2 mg PO for non-urgent INR elevation. Beware of drug interactions that potentiate (e.g., amiodarone, fluconazole) or inhibit (e.g., carbamazepine, rifampin) warfarin. Use with caution in hepatic impairment and renal disease. |
| Patient Advice | Take warfarin exactly as prescribed, at the same time each day. · Do not skip doses or double up if you miss a dose; contact your doctor for instructions. · Regular blood tests (INR) are required to ensure the dose is correct. · Avoid alcohol and consult doctor before starting or stopping any medications, including over-the-counter drugs and supplements. · Report any unusual bleeding or bruising, dark/tarry stools, blood in urine, coughing up blood, severe headache, or weakness immediately. · Use a soft toothbrush, electric razor, and be cautious with sharp objects to minimize bleeding risk. · Wear a medical alert bracelet and carry a card stating you take warfarin. · Do not change your diet significantly, especially foods high in vitamin K (e.g., spinach, kale, broccoli, Brussels sprouts) – maintain consistent intake. · If you are pregnant or plan to become pregnant, discuss with doctor; warfarin can harm the fetus. · Limit cranberry juice, green tea, and certain herbal supplements (e.g., ginseng, St. John's wort, ginkgo) as they may affect INR. |