INN monograph
Warfarin
Vitamin K antagonist anticoagulant · POM
Source-linked · Updated 03 Aug 2026 · Source: Local active-ingredient clinical extract; FDA drug label via OpenFDA/DailyMed; Anticoagulation standards / warfarin SmPC (batch 1); Professional class pharmacology (Anticoagulant)
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Risk first
Contraindications
- in: Pregnancy Warfarin sodium is contraindicated in women who are pregnant except in pregnant women with mechanical heart valves, who are at high risk of thromboembolism [see Warnings and Precautions ( 5.7 ) and Use in Specific Populations ( 8.1 )] .
- Warfarin sodium can cause fetal harm when administered to a pregnant woman.
- Warfarin sodium exposure during pregnancy causes a recognized pattern of major congenital malformations (warfarin embryopathy and fetotoxicity), fatal fetal hemorrhage, and an increased risk of spontaneous abortion and fetal mortality.
- If warfarin sodium is used during pregnancy or if the patient becomes pregnant while taking this drug, the patient should be apprised of the potential hazard to a fetus [see Use in Specific Populations ( 8.1 )] .
- Warfarin sodium is contraindicated in patients with: Hemorrhagic tendencies or blood dyscrasias Recent or contemplated surgery of the central nervous system or eye, or traumatic surgery resulting in large open surfaces [see Warnings and Precautions ( 5.8 )] Bleeding tendencies associated with: − Active ulceration or overt bleeding of the gastrointestinal, genitourinary, or respiratory tract − Central nervous system hemorrhage − Cerebral aneurysms, dissecting aorta − Pericarditis and pericardial effusions − Bacterial endocarditis Threatened abortion, eclampsia, and preeclampsia Unsupervised patients with conditions associated with potential high level of non-compliance Spinal puncture and other diagnostic or therapeutic procedures with potential for uncontrollable bleeding Hypersensitivity to warfarin or to any other components of this product (e.g., anaphylaxis) [see Adverse Reactions ( 6 )] Major regional or lumbar block anesthesia Malignant hypertension Pregnancy, except in women with mechanical heart valves ( 4 , 5.7 , 8.1 ) Hemorrhagic tendencies or blood dyscrasias ( 4 ) Recent or contemplated surgery of the central nervous system (CNS) or eye, or traumatic surgery resulting in large open surfaces ( 4 , 5.8 ) Bleeding tendencies associated with certain conditions ( 4 ) Threatened abortion, eclampsia, and preeclampsia ( 4 ) Unsupervised patients with potential high levels of non-compliance ( 4 ) Spinal puncture and other diagnostic or therapeutic procedures with potential for uncontrollable bleeding ( 4 ) Hypersensitivity to warfarin or any component of the product ( 4 ) Major regional or lumbar block anesthesia ( 4 ) Malignant hypertension ( 4 )
Precautions
- necrosis: Necrosis or gangrene of skin or other tissues can occur, with severe cases requiring debridement or amputation.
- Discontinue warfarin sodium and consider alternative anticoagulants if necessary.
- ( 5.2 ) Calciphylaxis: Fatal and serious cases have occurred.
- Discontinue warfarin sodium and consider alternative anticoagulation therapy.
- ( 5.3 ) Acute kidney injury may occur during episodes of excessive anticoagulation and hematuria.
- ( 5.4 ) Systemic atheroemboli and cholesterol microemboli: Some cases have progressed to necrosis or death.
- Discontinue warfarin sodium if such emboli occur.
- ( 5.5 ) Heparin-induced thrombocytopenia (HIT): Initial therapy with warfarin sodium in HIT has resulted in cases of amputation and death.
- Warfarin sodium may be considered after platelet count has normalized.
- ( 5.6 ) Pregnant women with mechanical heart valves: Warfarin sodium may cause fetal harm
- however, the benefits may outweigh the risks.
- ( 5.7 ) 5.1 Hemorrhage Warfarin sodium can cause major or fatal bleeding.
Point of care
Dosing
Adult
patient, and adjust based on INR response. ( 2.1 , 2.2 ) Knowledge of genotype can inform initial dose selection. ( 2.3 ) Monitoring: Obtain daily INR determinations upon initiation until stable in the therapeutic range. Obtain subsequent INR determinations every 1 to 4 weeks. ( 2.4 ) Review conversion instructions from other anticoagulants. ( 2.8 ) 2.1 Individualized Dosing The dosage and administration of warfarin sodium tablets must be individualized for each patient according to the patient’s International Normalized Ratio (INR) response to the drug. Adjust the dose based on the patient’s INR and the condition being treated. Consult the latest evidence-based clinical practice guidelines regarding the duration and intensity of anticoagulation for the indicated conditions. 2.2 Recommended Target INR Ranges and Durations for Individual Indications An INR of greater than 4 appears to provide no additional therapeutic benefit in most patients and is associated with a higher risk of bleeding. Venous Thromboembolism (including deep venous thrombosis [DVT] and PE) Adjust the warfarin dose to maintain a target INR of 2.5 (INR range, 2 to 3) for all treatment durations. The duration of treatment is based on the indication as follows: For patients with a DVT or PE secondary to a transient (reversible) risk factor, treatment with warfarin for 3 months is recommended. For patients with an unprovoked DVT or PE, treatment with warfarin is recommended for at least 3 months. After 3 months of therapy, evaluate the risk-benefit ratio of long-term treatment for the individual patient. For patients with two episodes of unprovoked DVT or PE, long-term treatment with warfarin is recommended. For a patient receiving long-term anticoagulant treatment, periodically reassess the risk-benefit ratio of continuing such treatment in the individual patient. Atrial Fibrillation In patients with non-valvular AF, anticoagulate with warfarin to target INR of 2.5 (range, 2 to 3). In patients with non-valvular AF that is persistent or paroxysmal and at high risk of stroke (i.e., having any of the following features: prior ischemic stroke, transient ischemic attack, or systemic embolism, or 2 of the following risk factors: age greater than 75 years, moderately or severely impaired left ventricular systolic function and/or heart failure, history of hypertension, or diabetes mellitus), long-term anticoagulation with warfarin is recommended. In patients with non-valvular AF that is persistent or paroxysmal and at an intermediate risk of ischemic stroke (i.e., having 1 of the following risk factors: age greater than 75 years, moderately or severely impaired left ventricular systolic function and/or heart failure, history of hypertension, or diabetes mellitus), long-term anticoagulation with warfarin is recommended. For patients with AF and mitral stenosis, long-term anticoagulation with warfarin is recommended. For patients with AF and prosthetic heart valves, long-term anticoagulation with warfarin is recommended; the target INR may be increased and aspirin added depending on valve type and position, and on patient factors. Mechanical and Bioprosthetic Heart Valves For patients with a bileaflet mechanical valve or a Medtronic Hall (Minneapolis, MN) tilting disk valve in the aortic position who are in sinus rhythm and without left atrial enlargement, therapy with warfarin to a target INR of 2.5 (range, 2 to 3) is recommended. For patients with tilting disk valves and bileaflet mechanical valves in the mitral position, therapy with warfarin to a target INR of 3 (range, 2.5 to 3.5) is recommended. For patients with caged ball or caged disk valves, therapy with warfarin to a target INR of 3 (range, 2.5 to 3.5) is recommended. For patients with a bioprosthetic valve in the mitral position, therapy with warfarin to a target INR of 2.5 (range, 2 to 3) for the first 3 months after valve insertion is recommended. If additional risk factors for thromboembolism are present (AF, previous thromboembolism, left ventricular dysfunction), a target INR of 2.5 (range 2 to 3) is recommended. Post-Myocardial Infarction For high-risk patients with MI (e.g., those with a large anterior MI, those with significant heart failure, those with intracardiac thrombus visible on transthoracic echocardiography, those with AF, and those with a history of a thromboembolic event), therapy with combined moderate-intensity (INR, 2 to 3) warfarin plus low-dose aspirin (≤ 100 mg/day) for at least 3 months after the MI is recommended. Recurrent Systemic Embolism and Other Indications Oral anticoagulation therapy with warfarin has not been fully evaluated by clinical trials in patients with valvular disease associated with AF, patients with mitral stenosis, and patients with recurrent systemic embolism of unknown etiology. However, a moderate dose regimen (INR 2 to 3) may be used for these patients. 2.3 Initial and Maintenance Dosing The appropriate initial dosing of warfarin sodium tablets varies widely for different patients. Not all factors responsible for warfarin dose variability are known, and the initial dose is influenced by: Clinical factors including age, race, body weight, sex, concomitant medications, and comorbidities Genetic factors (CYP2C9 and VKORC1 genotypes) [see Clinical Pharmacology ( 12.5 )] Select the initial dose based on the expected maintenance dose, taking into account the above factors. Modify this dose based on consideration of patient-specific clinical factors. Consider lower initial and maintenance doses for elderly and/or debilitated patients and in Asian patients [see Use in Specific Populations ( 8.5 ) and Clinical Pharmacology ( 12.3 )] . Routine use of loading doses is not recommended as this practice may increase hemorrhagic and other complications and does not offer more rapid protection against clot formation. Individualize the duration of therapy for each patient. In general, anticoagulant therapy should be continued until the danger of thrombosis and embolism has passed [see Dosage and Administration ( 2.2 )] . Dosing Recommendations without Consideration of Genotype If the patient’s CYP2C9 and VKORC1 genotypes are not known, the initial dose of warfarin sodium tablets is usually 2 to 5 mg once daily. Determine each patient’s dosing needs by close monitoring of the INR response and consideration of the indication being treated. Typical maintenance doses are 2 to 10 mg once daily. Dosing Recommendations with Consideration of Genotype Table 1 displays three ranges of expected maintenance warfarin sodium tablets doses observed in subgroups of patients having different combinations of CYP2C9 and VKORC1 gene variants [see Clinical Pharmacology ( 12.5 )] . If the patient’s CYP2C9 and/or VKORC1 genotype are known, consider these ranges in choosing the initial dose. Patients with CYP2C9 *1/*3, *2/*2, *2/*3, and *3/*3 may require more prolonged time (> 2 to 4 weeks) to achieve maximum INR effect for a given dosage regimen than patients without these CYP variants. Table 1: Three Ranges of Expected Maintenance Warfarin Sodium Tablets Daily Doses Based on CYP2C9 and VKORC1 Genotypes † VKORC1 CYP2C9 *1/*1 *1/*2 *1/*3 *2/*2 *2/*3 *3/*3 GG 5 to 7 mg 5 to 7 mg 3 to 4 mg 3 to 4 mg 3 to 4 mg 0.5 to 2 mg AG 5 to 7 mg 3 to 4 mg 3 to 4 mg 3 to 4 mg 0.5 to 2 mg 0.5 to 2 mg AA 3 to 4 mg 3 to 4 mg 0.5 to 2 mg 0.5 to 2 mg 0.5 to 2 mg 0.5 to 2 mg † Ranges are derived from multiple published clinical studies. VKORC1−1639G > A (rs9923231) variant is used in this table. Other co-inherited VKORC1 variants may also be important determinants of warfarin dose. 2.4 Monitoring to Achieve Optimal Anticoagulation Warfarin sodium tablets have a narrow therapeutic range (index), and their action may be affected by factors such as other drugs and dietary vitamin K. Therefore, anticoagulation must be carefully monitored during warfarin sodium tablets therapy. Determine the INR daily after the administration of the initial dose until INR results stabilize in the therapeutic range. After stabilization, maintain dosing within the therapeutic range by performing periodic INRs. The frequency of performing INR should be based on the clinical situation but generally acceptable intervals for INR determinations are 1 to 4 weeks. Perform additional INR tests when other warfarin products are interchanged with warfarin sodium tablets, as well as whenever other medications are initiated, discontinued, or taken irregularly. Heparin, a common concomitant drug, increases the INR [see Dosage and Administration ( 2.8 ) and Drug Interactions ( 7 )] . Determinations of whole blood clotting and bleeding times are not effective measures for monitoring of warfarin sodium tablets therapy. 2.5 Renal Impairment No dosage adjustment is necessary for patients with renal failure. Monitor INR more frequently in patients with compromised renal function to maintain INR within the therapeutic range [see Warnings and Precautions ( 5.4 ) and Use in Specific Populations ( 8.6 )]. 2.6 Missed Dose The anticoagulant effect of warfarin sodium tablets persists beyond 24 hours. If a patient misses a dose of warfarin sodium tablets at the intended time of day, the patient should take the dose as soon as possible on the same day. The patient should not double the dose the next day to make up for a missed dose. 2.7 Treatment During Dentistry and Surgery Some dental or surgical procedures may necessitate the interruption or change in the dose of warfarin sodium tablets therapy. Consider the benefits and risks when discontinuing warfarin sodium tablets even for a short period of time. Determine the INR immediately prior to any dental or surgical procedure. In patients undergoing minimally invasive procedures who must be anticoagulated prior to, during, or immediately following these procedures, adjusting the dosage of warfarin sodium tablets to maintain the INR at the low end of the therapeutic range may safely allow for continued anticoagulation. 2.8 Conversion From Other Anticoagulants Heparin Since the full anticoagulant effect of warfarin sodium tablets is not achieved for several days, heparin is preferred for initial rapid anticoagulation. During initial therapy with warfarin sodium tablets, the interference with heparin anticoagulation is of minimal clinical significance. Conversion to warfarin sodium tablets may begin concomitantly with heparin therapy or may be delayed 3 to 6 days. To ensure therapeutic anticoagulation, continue full dose heparin therapy and overlap warfarin sodium tablets therapy with heparin for 4 to 5 days and until warfarin sodium tablets has produced the desired therapeutic response as determined by INR, at which point heparin may be discontinued. As heparin may affect the INR, patients receiving both heparin and warfarin sodium tablets should have INR monitoring at least: 5 hours after the last intravenous bolus dose of heparin, or 4 hours after cessation of a continuous intravenous infusion of heparin, or 24 hours after the last subcutaneous heparin injection. Warfarin sodium tablets may increase the activated partial thromboplastin time (aPTT) test, even in the absence of heparin. A severe elevation (> 50 seconds) in aPTT with an INR in the desired range has been identified as an indication of increased risk of postoperative hemorrhage. Other Anticoagulants Consult the labeling of other anticoagulants for instructions on conversion to warfarin sodium tablets.
Paediatric
8.4 Pediatric Use Adequate and well-controlled studies with warfarin sodium have not been conducted in any pediatric population, and the optimum dosing, safety, and efficacy in pediatric patients is unknown. Pediatric use of warfarin sodium is based on adult data and recommendations, and available limited pediatric data from observational studies and patient registries. Pediatric patients administered warfarin sodium should avoid any activity or sport that may result in traumatic injury. The developing hemostatic system in infants and children results in a changing physiology of thrombosis and response to anticoagulants. Dosing of warfarin in the pediatric population varies by patient age, with infants generally having the highest, and adolescents having the lowest milligram per kilogram dose requirements to maintain target INRs. Because of changing warfarin requirements due to age, concomitant medications, diet, and existing medical condition, target INR ranges may be difficult to achieve and maintain in pediatric patients, and more frequent INR determinations are recommended. Bleeding rates varied by patient population and clinical care center in pediatric observational studies and patient registries. Infants and children receiving vitamin K-supplemented nutrition, including infant formulas, may be resistant to warfarin therapy, while human milk-fed infants may be sensitive to warfarin therapy.
Renal
Not renally cleared primarily, but CKD patients need careful INR monitoring.
- CrCl 0–120: Dose by INR, not CrCl; monitor more closely in CKD.
Hepatic
Impaired synthesis of clotting factors — use extreme caution; lower doses.
Safety
Drug interactions
- ISMP high-alert.
- Document indication, target INR (warfarin), planned duration.
- Have reversal strategy accessible (vitamin K/PCC, idarucizumab, andexanet where available, protamine for UFH).
Safety
Adverse effects
- labeling: Hemorrhage [see Boxed Warning, Warnings and Precautions (5.1), and Overdosage ( 10 )] Tissue Necrosis [see Warnings and Precautions ( 5.2 )] Calciphylaxis [see Warnings and Precautions ( 5.3 )] Acute Kidney Injury [see Warnings and Precautions ( 5.4 )] Systemic Atheroemboli and Cholesterol Microemboli [see Warnings and Precautions ( 5.5 )] Limb Ischemia, Necrosis, and Gangrene in Patients with HIT and HITTS [see Warnings and Precautions ( 5.6 )] Other Clinical Settings with Increased Risks [see Warnings and Precautions ( 5.8 )] Other adverse reactions to warfarin sodium include: Immune system disorders: hypersensitivity/allergic reactions (including urticaria and anaphylactic reactions) Vascular disorders: vasculitis Hepatobiliary disorders: hepatitis, elevated liver enzymes.
- Cholestatic hepatitis has been associated with concomitant administration of warfarin sodium and ticlopidine.
- Gastrointestinal disorders: nausea, vomiting, diarrhea, taste perversion, abdominal pain, flatulence, bloating Skin disorders: rash, dermatitis (including bullous eruptions), pruritus, alopecia Respiratory disorders: tracheal or tracheobronchial calcification General disorders: chills Most common adverse reactions to warfarin sodium are fatal and nonfatal hemorrhage from any tissue or organ.
- ( 6 ) To report SUSPECTED ADVERSE REACTIONS, contact Teva at 1-888-838-2872 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch.
Use
Indications
- for: Prophylaxis and treatment of venous thrombosis and its extension, pulmonary embolism (PE).
- Prophylaxis and treatment of thromboembolic complications associated with atrial fibrillation (AF) and/or cardiac valve replacement.
- Reduction in the risk of death, recurrent myocardial infarction (MI), and thromboembolic events such as stroke or systemic embolization after myocardial infarction.
- Limitations of Use Warfarin sodium tablets have no direct effect on an established thrombus, nor does it reverse ischemic tissue damage.
- Once a thrombus has occurred, however, the goals of anticoagulant treatment are to prevent further extension of the formed clot and to prevent secondary thromboembolic complications that may result in serious and possibly fatal sequelae.
- Warfarin sodium tablets are a vitamin K antagonist indicated for: Prophylaxis and treatment of venous thrombosis and its extension, pulmonary embolism ( 1 ) Prophylaxis and treatment of thromboembolic complications associated with atrial fibrillation and/or cardiac valve replacement ( 1 ) Reduction in the risk of death, recurrent myocardial infarction, and thromboembolic events such as stroke or systemic embolization after myocardial infarction ( 1 ) Limitations of Use Warfarin sodium tablets have no direct effect on an established thrombus, nor does it reverse ischemic tissue damage.
Pharmacology
Mode of action
factors, which include Factors II, VII, IX, and X, and the anticoagulant proteins C and S.
Full mechanism text
factors, which include Factors II, VII, IX, and X, and the anticoagulant proteins C and S. Vitamin K is an essential cofactor for the post ribosomal synthesis of the vitamin K-dependent clotting factors. Vitamin K promotes the biosynthesis of γ-carboxyglutamic acid residues in the proteins that are essential for biological activity. Warfarin is thought to interfere with clotting factor synthesis by inhibition of the C1 subunit of vitamin K epoxide reductase (VKORC1) enzyme complex, thereby reducing the regeneration of vitamin K 1 epoxide [see Clinical Pharmacology ( 12.5 )] .
ADME
Pharmacokinetics & PD
| Onset | Anticoagulant effect over 2–3 days (existing factors must clear) |
|---|---|
| Duration | Days after stopping (factor resynthesis) |
| Route | ORAL / SC / IV |
| Absorption | Warfarin is essentially completely absorbed after oral administration, with peak concentration generally attained within the first 4 hours. |
| Distribution | Warfarin shows a volume of distribution of about 0.14 L/kg. Approximately 99% of the drug is bound to plasma proteins. |
| Elimination | of warfarin is almost entirely by |
| Half-life | of warfarin after a single dose is approximately 1 week; however, the effective half-life ranges from 20 to 60 hours, with a mean of about 40 hours. The clearance of R-warfarin is generally half that of S-warfarin, thus as the volumes of |
Full PK/PD text
12 CLINICAL PHARMACOLOGY 12.1 Mechanism of Action Warfarin acts by inhibiting the synthesis of vitamin K-dependent clotting factors, which include Factors II, VII, IX, and X, and the anticoagulant proteins C and S. Vitamin K is an essential cofactor for the post ribosomal synthesis of the vitamin K-dependent clotting factors. Vitamin K promotes the biosynthesis of γ-carboxyglutamic acid residues in the proteins that are essential for biological activity. Warfarin is thought to interfere with clotting factor synthesis by inhibition of the C1 subunit of vitamin K epoxide reductase (VKORC1) enzyme complex, thereby reducing the regeneration of vitamin K 1 epoxide [see Clinical Pharmacology ( 12.5 )] . 12.2 Pharmacodynamics An anticoagulation effect generally occurs within 24 hours after warfarin administration. However, peak anticoagulant effect may be delayed 72 to 96 hours. The duration of action of a single dose of racemic warfarin is 2 to 5 days. The effects of warfarin sodium may become more pronounced as effects of daily maintenance doses overlap. This is consistent with the half-lives of the affected vitamin K-dependent clotting factors and anticoagulation proteins: Factor II - 60 hours, VII - 4 to 6 hours, IX - 24 hours, X - 48 to 72 hours, and proteins C and S are approximately 8 hours and 30 hours, respectively. 12.3 Pharmacokinetics Warfarin sodium is a racemic mixture of the R - and S -enantiomers of warfarin. The S -enantiomer exhibits 2 to 5 times more anticoagulant activity than the R -enantiomer in humans, but generally has a more rapid clearance. Absorption Warfarin is essentially completely absorbed after oral administration, with peak concentration generally attained within the first 4 hours. Distribution Warfarin shows a volume of distribution of about 0.14 L/kg. Approximately 99% of the drug is bound to plasma proteins. Metabolism The elimination of warfarin is almost entirely by metabolism. Warfarin is stereoselectively metabolized by hepatic cytochrome P-450 (CYP450) microsomal enzymes to inactive hydroxylated metabolites (predominant route) and by reductases to reduced metabolites (warfarin alcohols) with minimal anticoagulant activity. Identified metabolites of warfarin include dehydrowarfarin, two diastereoisomer alcohols, and 4′-, 6-, 7-, 8-, and 10- hydroxywarfarin. The CYP450 isozymes involved in the metabolism of warfarin include CYP2C9, 2C19, 2C8, 2C18, 1A2, and 3A4. CYP2C9, a polymorphic enzyme, is likely to be the principal form of human liver CYP450 that modulates the in vivo anticoagulant activity of warfarin. Patients with one or more variant CYP2C9 alleles have decreased S-warfarin clearance [see Clinical Pharmacology ( 12.5 )] . Excretion The terminal half-life of warfarin after a single dose is approximately 1 week; however, the effective half-life ranges from 20 to 60 hours, with a mean of about 40 hours. The clearance of R-warfarin is generally half that of S-warfarin, thus as the volumes of distribution are similar, the half-life of R-warfarin is longer than that of S-warfarin. The half-life of R-warfarin ranges from 37 to 89 hours, while that of S-warfarin ranges from 21 to 43 hours. Studies with radiolabeled drug have demonstrated that up to 92% of the orally administered dose is recovered in urine. Very little warfarin is excreted unchanged in urine. Urinary excretion is in the form of metabolites. Geriatric Patients Patients 60 years or older appear to exhibit greater than expected INR response to the anticoagulant effects of warfarin. The cause of the increased sensitivity to the anticoagulant effects of warfarin in this age group is unknown but may be due to a combination of pharmacokinetic and pharmacodynamic factors. Limited information suggests there is no difference in the clearance of S-warfarin; however, there may be a slight decrease in the clearance of R-warfarin in the elderly as compared to the young. Therefore, as patient age increases, a lower dose of warfarin is usually required to produce a therapeutic level of anticoagulation [see Dosage and Administration ( 2.3 , 2.4 )]. Asian Patients Asian patients may require lower initiation and maintenance doses of warfarin. A non-controlled study of 151 Chinese outpatients stabilized on warfarin for various indications reported a mean daily warfarin requirement of 3.3 ± 1.4 mg to achieve an INR of 2 to 2.5. Patient age was the most important determinant of warfarin requirement in these patients, with a progressively lower warfarin requirement with increasing age. 12.5 Pharmacogenomics CYP2C9 and VKORC1 Polymorphisms The S -enantiomer of warfarin is mainly metabolized to 7-hydroxywarfarin by CYP2C9, a polymorphic enzyme. The variant alleles, CYP2C9*2 and CYP2C9*3, result in decreased in vitro CYP2C9 enzymatic 7-hydroxylation of S-warfarin. The frequencies of these alleles in Caucasians are approximately 11% and 7% for CYP2C9*2 and CYP2C9*3, respectively. Other CYP2C9 alleles associated with reduced enzymatic activity occur at lower frequencies, including *5, *6, and *11 alleles in populations of African ancestry and *5, *9, and *11 alleles in Caucasians. Warfarin reduces the regeneration of vitamin K from vitamin K epoxide in the vitamin K cycle through inhibition of VKOR, a multiprotein enzyme complex. Certain single nucleotide polymorphisms in the VKORC1 gene (e.g., –1639G > A) have been associated with variable warfarin dose requirements. VKORC1 and CYP2C9 gene variants generally explain the largest proportion of known variability in warfarin dose requirements. CYP2C9 and VKORC1 genotype information, when available, can assist in selection of the initial dose of warfarin [see Dosage and Administration ( 2.3 )] .
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Special populations
Pregnancy & lactation
Pregnancy
8.1 Pregnancy Risk Summary Warfarin sodium is contraindicated in women who are pregnant except in pregnant women with mechanical heart valves, who are at high risk of thromboembolism, and for whom the benefits of warfarin sodium may outweigh the risks [see Warnings and Precautions ( 5.7 )] . Warfarin sodium can cause fetal harm. Exposure to warfarin during the first trimester of pregnancy caused a pattern of congenital malformations in about 5% of exposed offspring. Because these data were not collected in adequate and well-controlled studies, this incidence of major birth defects is not an adequate basis for comparison to the estimated incidences in the control group or the U.S. general population and may not reflect the incidences observed in practice. Consider the benefits and risks of warfarin sodium and possible risks to the fetus when prescribing warfarin sodium to a pregnant woman. Adverse outcomes in pregnancy occur regardless of the health of the mother or the use of medications. The estimated background risk of major birth defects and miscarriage for the indicated population is unknown. In the U.S. general population, the estimated background risk of major birth defects and miscarriage in clinically recognized pregnancies is 2% to 4% and 15% to 20%, respectively. Clinical Considerations Fetal/Neonatal Adverse Reactions In humans, warfarin crosses the placenta, and concentrations in fetal plasma approach the maternal values. Exposure to warfarin during the first trimester of pregnancy caused a pattern of congenital malformations in about 5% of exposed offspring. Warfarin embryopathy is characterized by nasal hypoplasia with or without stippled epiphyses (chondrodysplasia punctata) and growth retardation (including low birth weight). Central nervous system and eye abnormalities have also been reported, including dorsal midline dysplasia characterized by agenesis of the corpus callosum, Dandy-Walker malformation, midline cerebellar atrophy, and ventral midline dysplasia characterized by optic atrophy. Mental retardation, blindness, schizencephaly, microcephaly, hydrocephalus, and other adverse pregnancy outcomes have been reported following warfarin exposure during the second and third trimesters of pregnancy [see Contraindications ( 4 )] .
Lactation
valves: Warfarin sodium may cause fetal harm; however, the benefits may outweigh the risks. ( 8.1 ) Lactation: Monitor breastfeeding infants for bruising or bleeding. ( 8.2 ) Renal Impairment: Instruct patients with renal impairment to frequently monitor their INR. ( 8.6 ) 8.1 Pregnancy Risk Summary Warfarin sodium is contraindicated in women who are pregnant except in pregnant women with mechanical heart valves, who are at high risk of thromboembolism, and for whom the benefits of warfarin sodium may outweigh the risks [see Warnings and Precautions ( 5.7 )] . Warfarin sodium can cause fetal harm. Exposure to warfarin during the first trimester of pregnancy caused a pattern of congenital malformations in about 5% of exposed offspring. Because these data were not collected in adequate and well-controlled studies, this incidence of major birth defects is not an adequate basis for comparison to the estimated incidences in the control group or the U.S. general population and may not reflect the incidences observed in practice. Consider the benefits and risks of warfarin sodium and possible risks to the fetus when prescribing warfarin sodium to a pregnant woman. Adverse outcomes in pregnancy occur regardless of the health of the mother or the use of medications. The estimated background risk of major birth defects and miscarriage for the indicated population is unknown. In the U.S. general population, the estimated background risk of major birth defects and miscarriage in clinically recognized pregnancies is 2% to 4% and 15% to 20%, respectively. Clinical Considerations Fetal/Neonatal Adverse Reactions In humans, warfarin crosses the placenta, and concentrations in fetal plasma approach the maternal values. Exposure to warfarin during the first trimester of pregnancy caused a pattern of congenital malformations in about 5% of exposed offspring. Warfarin embryopathy is characterized by nasal hypoplasia with or without stippled epiphyses (chondrodysplasia punctata) and growth retardation (including low birth weight). Central nervous system and eye abnormalities have also been reported, including dorsal midline dysplasia characterized by agenesis of the corpus callosum, Dandy-Walker malformation, midline cerebellar atrophy, and ventral midline dysplasia characterized by optic atrophy. Mental retardation, blindness, schizencephaly, microcephaly, hydrocephalus, and other adverse pregnancy outcomes have been reported following warfarin exposure during the second and third trimesters of pregnancy [see Contraindications ( 4 )] . 8.2 Lactation Risk Summary Warfarin was not present in human milk from mothers treated with warfarin from a limited published study. Because of the potential for serious adverse reactions, including bleeding in a breastfed infant, consider the developmental and health benefits of breastfeeding along with the mother’s clinical need for warfarin sodium and any potential adverse effects on the breastfed infant from warfarin sodium or from the underlying maternal condition before prescribing warfarin sodium to a lactating woman. Clinical Considerations Monitor breastfeeding infants for bruising or bleeding. Data Human Data Based on published data in 15 nursing mothers, warfarin was not detected in human milk. Among the 15 full-term newborns, 6 nursing infants had documented prothrombin times within the expected range. Prothrombin times were not obtained for the other 9 nursing infants. Effects in premature infants have not been evaluated. 8.3 Females and Males of Reproductive Potential Pregnancy Testing Warfarin sodium can cause fetal harm [see Use in Specific Populations ( 8.1 )] . Verify the pregnancy status of females of reproductive potential prior to initiating warfarin sodium therapy. Contraception Females Advise females of reproductive potential to use effective contraception during treatment and for at least 1 month after the final dose of warfarin sodium. 8.4 Pediatric Use Adequate and well-controlled studies with warfarin sodium have not been conducted in any pediatric population, and the optimum dosing, safety, and efficacy in pediatric patients is unknown. Pediatric use of warfarin sodium is based on adult data and recommendations, and available limited pediatric data from observational studies and patient registries. Pediatric patients administered warfarin sodium should avoid any activity or sport that may result in traumatic injury. The developing hemostatic system in infants and children results in a changing physiology of thrombosis and response to anticoagulants. Dosing of warfarin in the pediatric population varies by patient age, with infants generally having the highest, and adolescents having the lowest milligram per kilogram dose requirements to maintain target INRs. Because of changing warfarin requirements due to age, concomitant medications, diet, and existing medical condition, target INR ranges may be difficult to achieve and maintain in pediatric patients, and more frequent INR determinations are recommended. Bleeding rates varied by patient population and clinical care center in pediatric observational studies and patient registries. Infants and children receiving vitamin K-supplemented nutrition, including infant formulas, may be resistant to warfarin therapy, while human milk-fed infants may be sensitive to warfarin therapy. 8.5 Geriatric Use Of the total number of patients receiving warfarin sodium in controlled clinical trials for which data were available for analysis, 1885 patients (24.4%) were 65 years and older, while 185 patients (2.4%) were 75 years and older. No overall differences in effectiveness or safety were observed between these patients and younger patients, but greater sensitivity of some older individuals cannot be ruled out. Patients 60 years or older appear to exhibit greater than expected INR response to the anticoagulant effects of warfarin [see Clinical Pharmacology ( 12.3 )] . Warfarin sodium is contraindicated in any unsupervised patient with senility. Conduct more frequent monitoring for bleeding with administration of warfarin sodium to elderly patients in any situation or with any physical condition where added risk of hemorrhage is present. Consider lower initiation and maintenance doses of warfarin sodium in elderly patients [see Dosage and Administration ( 2.2 , 2.3 )] . 8.6 Renal Impairment Renal clearance is considered to be a minor determinant of anticoagulant response to warfarin. No dosage adjustment is necessary for patients with renal impairment. Instruct patients with renal impairment taking warfarin to monitor their INR more frequently [see Warnings and Precautions ( 5.4 )] . 8.7 Hepatic Impairment Hepatic impairment can potentiate the response to warfarin through impaired synthesis of clotting factors and decreased metabolism of warfarin. Conduct more frequent monitoring for bleeding when using warfarin sodium in these patients.
Diet
Food & alcohol
- Drug & food interactions (label) 7 DRUG INTERACTIONS Concomitant use of drugs that increase bleeding risk, antibiotics, antifungals, botanical (herbal) products, and inhibitors and inducers of CYP2C9, 1A2, or 3A4. ( 7 ) Consult labeling of all concurrently used drugs for complete information about interactions with warfarin sodium or increased risks for bleeding. ( 7 ) 7.1 General Information Drugs may interact with warfarin sodium through pharmacodynamic or pharmacokinetic mechanisms. Pharmacodynamic mechanisms for drug interactions with warfarin sodium are synergism (impaired hemostasis, reduced clotting factor synthesis), competitive antagonism (vitamin K), and alteration of the physiologic control loop for vitamin K metabolism (hereditary resistance). Pharmacokinetic mechanisms for drug interactions with warfarin sodium are mainly enzyme induction, enzyme inhibition, and reduced plasma protein binding. It is important to note that some drugs may interact by more than one mechanism. More frequent INR monitoring should be performed when starting or stopping other drugs, including botanicals, or when changing dosages of other drugs, including drugs intended for short-term use (e.g., antibiotics, antifungals, corticosteroids) [ see Boxed Warning ]. Consult the labeling of all concurrently used drugs to obtain further information about interactions with warfarin sodium or adverse reactions pertaining to bleeding. 7.2 CYP450 Interactions CYP450 isozymes involved in the metabolism of warfarin include CYP2C9, 2C19, 2C8, 2C18, 1A2, and 3A4. The more potent warfarin S -enantiomer is metabolized by CYP2C9 while the R -enantiomer is metabolized by CYP1A2 and 3A4. Inhibitors of CYP2C9, 1A2, and/or 3A4 have the potential to increase the effect (increase INR) of warfarin by increasing the exposure of warfarin. Inducers of CYP2C9, 1A2, and/or 3A4 have the potential to decrease the effect (decrease INR) of warfarin by decreasing the exposure of warfarin. Examples of inhibitors and inducers of CYP2C9, 1A2, and 3A4 are below in Table 2 ; however, this list should not be considered all-inclusive. Consult the labeling of all concurrently used drugs to obtain further information about CYP450 interaction potential. The CYP450 inhibition and induction potential should be considered when starting, stopping, or changing dose of concomitant medications. Closely monitor INR if a concomitant drug is a CYP2C9, 1A2, and/or 3A4 inhibitor or inducer. Table 2: Examples of CYP450 Interactions with Warfarin Enzyme Inhibitors Inducers CYP2C9 amiodarone, capecitabine, cotrimoxazole, etravirine, fluconazole, fluvastatin, fluvoxamine, metronidazole, miconazole, oxandrolone, sulfinpyrazone, tigecycline, voriconazole, zafirlukast aprepitant, bosentan, carbamazepine, phenobarbital, rifampin CYP1A2 acyclovir, allopurinol, caffeine, cimetidine, ciprofloxacin, disulfiram, enoxacin, famotidine, fluvoxamine, methoxsalen, mexiletine, norfloxacin, oral contraceptives, phenylpropanolamine, propafenone, propranolol, terbinafi
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Sources & disclaimer
Source: Local active-ingredient clinical extract; FDA drug label via OpenFDA/DailyMed; Anticoagulation standards / warfarin SmPC (batch 1); Professional class pharmacology (Anticoagulant)
Clinical review date not recorded.
Decision support only — not a substitute for clinical judgment, product SmPC, or Kenya STG/EML.