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All about: Warfarin

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Generic Name: Warfarin sodium
Dosage Form: Tablets, usp


Rx only


Warfarin sodium is an anticoagulant which acts by inhibiting vitamin K-dependent coagulation factors. Chemically, it is 3-(α-acetonylbenzyl)-4-hydroxycoumarin and is a racemic mixture of the R and S enantiomers. Warfarin sodium is an isopropanol clathrate. The crystallization of Warfarin sodium virtually eliminates trace impurities present in amorphous Warfarin. Its structural formula may be represented as follows:

C19H15NaO4 Molecular Weight: 330.31

Warfarin sodium occurs as a white, odorless, crystalline powder, is discolored by light and is very soluble in water; freely soluble in alcohol; very slightly soluble in chloroform and in ether.

Each tablet, for oral administration, contains 1 mg, 2 mg, 2½ mg, 3 mg, 4 mg, 5 mg, 6 mg, 7½ mg or 10 mg Warfarin sodium. In addition, each tablet contains the following inactive ingredients: Anhydrous lactose, hypromellose 2208 (3cPs), magnesium stearate, and pregelatinized starch.

The 1 mg also contains D&C red no. 6 barium lake.

The 2 mg also contains FD&C blue no. 2 aluminum lake, and FD&C red no. 40 aluminum lake.

The 2½ mg also contains D&C yellow no. 10 aluminum lake, and FD&C blue no. 1 aluminum lake.

The 3 mg also contains FD&C blue no. 2 aluminum lake, FD&C red no. 40 aluminum lake, and FD&C yellow no. 6 aluminum lake.

The 4 mg also contains FD&C blue no. 1 aluminum lake.

The 5 mg also contains FD&C yellow no. 6 aluminum lake.

The 6 mg also contains FD&C blue no. 1 aluminum lake, and FD&C yellow no. 6 aluminum lake.

The 7½ mg also contains D&C yellow no. 10 aluminum lake, and FD&C yellow no. 6 aluminum lake.

The 10 mg does not contain any dyes.


Warfarin sodium and other coumarin anticoagulants act 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. Half-lives of these clotting factors are as follows: Factor II - 60 hours, VII - 4 to 6 hours, IX - 24 hours, and X - 48 to 72 hours. The half-lives of proteins C and S are approximately 8 hours and 30 hours, respectively. The resultant in vivo effect is a sequential depression of Factors VII, IX, X, and II activities. Vitamin K is an essential cofactor for the post ribosomal synthesis of the vitamin K dependent clotting factors. The vitamin promotes the biosynthesis of Υ-carboxyglutamic acid residues in the proteins which are essential for biological activity. Warfarin is thought to interfere with clotting factor synthesis by inhibition of the regeneration of vitamin K1 epoxide. The degree of depression is dependent upon the dosage administered. Therapeutic doses of Warfarin decrease the total amount of the active form of each vitamin K dependent clotting factor made by the liver by approximately 30% to 50%.

An anticoagulation effect generally occurs within 24 hours after drug 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. Anticoagulants have no direct effect on an established thrombus, nor do they reverse ischemic tissue damage. However, once a thrombus has occurred, the goal of anticoagulant treatment is to prevent further extension of the formed clot and prevent secondary thromboembolic complications which may result in serious and possibly fatal sequelae.


Warfarin sodium is a racemic mixture of the R- and S-enantiomers. The S-enantiomer exhibits 2 to 5 times more anticoagulant activity than the R-enantiomer in humans, but generally has a more rapid clearance.


Warfarin sodium is essentially completely absorbed after oral administration with peak concentration generally attained within the first 4 hours.


There are no differences in the apparent volumes of distribution after intravenous and oral administration of single doses of Warfarin solution. Warfarin distributes into a relatively small apparent volume of distribution of about 0.14 liter/kg. Warfarin distributes into a phase lasting 6 to 12 hours is distinguishable after rapid intravenous or oral administration of an aqueous solution. Using a one compartment model, and assuming complete bioavailability, estimates of the volumes of distribution of R- and S-Warfarin are similar to each other and to that of the racemate. Concentrations in fetal plasma approach the maternal values, but Warfarin has not been found in human milk (see WARNINGS, Lactation). Approximately 99% of the drug is bound to plasma proteins.


The elimination of Warfarin is almost entirely by metabolism. Warfarin sodium is stereoselectively metabolized by hepatic microsomal enzymes (cytochrome P-450) to inactive hydroxylated metabolites (predominant route) and by reductases to reduced metabolites (Warfarin alcohols). The Warfarin alcohols have minimal anticoagulant activity. The metabolites are principally excreted into the urine; and to a lesser extent into the bile. The metabolites of Warfarin that have been identified include dehydroWarfarin, two diastereoisomer alcohols, 4’-, 6-, 7-, 8- and 10-hydroxyWarfarin. The Cytochrome P-450 isozymes involved in the metabolism of Warfarin include 2C9, 2C19, 2C8, 2C18, 1A2, and 3A4. 2C9 is likely to be the principal form of human liver P-450 which modulates the in vivo anticoagulant activity of Warfarin.


The terminal half-life of Warfarin after a single dose is approximately one 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.


Patients 60 years or older appear to exhibit greater than expected PT/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. This increased anticoagulant effect from Warfarin may be due to a combination of pharmacokinetic and pharmacodynamic factors. Racemic Warfarin clearance may be unchanged or reduced with increasing age. Limited information suggests there is no difference in the clearance of S-Warfarin in the elderly versus young subjects. 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.


Asian patients may require lower initiation and maintenance doses of Warfarin. One non-controlled study conducted in 151 Chinese outpatients reported a mean daily Warfarin requirement of 3.3 ± 1.4 mg to achieve an INR of 2 to 2.5. These patients were stabilized on Warfarin for various indications. Patient age was the most important determinant of Warfarin requirement in Chinese patients with a progressively lower Warfarin requirement with increasing age.

Renal Dysfunction:

Renal clearance is considered to be a minor determinant of anticoagulant response to Warfarin. No dosage adjustment is necessary for patients with renal failure.

Hepatic Dysfunction:

Hepatic dysfunction can potentiate the response to Warfarin through impaired synthesis of clotting factors and decreased metabolism of Warfarin.

The administration of Warfarin sodium via the intravenous (I.V.) route should provide the patient with the same concentration of an equal oral dose, but maximum plasma concentration will be reached earlier. However, the full anticoagulant effect of a dose of Warfarin may not be achieved until 72 to 96 hours after dosing, indicating that the administration of I.V. Warfarin sodium should not provide any increased biological effect or earlier onset of action.

Clinical Trials:

Atrial Fibrillation (AF):

In five prospective randomized controlled clinical trials involving 3711 patients with non-rheumatic AF, Warfarin significantly reduced the risk of systemic thromboembolism including stroke (see Table 1). The risk reduction ranged from 60% to 86% in all except one trial (CAFA: 45%) which stopped early due to published positive results from two of these trials. The incidence of major bleeding in these trials ranged from 0.6 to 2.7% (see Table 1). Meta-analysis findings of these studies revealed that the effects of Warfarin in reducing thromboembolic events including stroke were similar at either moderately high INR (2.0 to 4.5) or low INR (1.4 to 3.0). There was a significant reduction in minor bleeds at the low INR. Similar data from clinical studies in valvular atrial fibrillation patients are not available.

*All study results of Warfarin vs. control are based on intention-to-treat analysis and include ischemic stroke and systemic thromboembolism, excluding hemorrhage and transient ischemic attacks.
Study N PT Ratio INR Thromboembolism % Major Bleeding
Warfarin-Treated Patients Control Patients % Risk Reduction p value Warfarin- Treated Patients Control Patients
AFASAK 335 336 1.5-2.0 2.8-4.2 60 0.027 0.6 0.0
SPAF 210 211 1.3-1.8 2.0-4.5 67 0.01 1.9 1.9
BAATAF 212 208 1.2-1.5 1.5-2.7 86 <0.05 0.9 0.5
CAFA 187 191 1.3-1.6 2.0-3.0 45 0.25 2.7 0.5
SPINAF 260 265 1.2-1.5 1.4-2.8 79 0.001 2.3 1.5

Myocardial Infarction:

WARIS (The Warfarin Re-Infarction Study) was a double-blind, randomized study of 1214 patients 2 to 4 weeks post-infarction treated with Warfarin to a target INR of 2.8 to 4.8. [But note that a lower INR was achieved and increased bleeding was associated with INR’s above 4.0; (see DOSAGE AND ADMINISTRATION)]. The primary endpoint was a combination of total mortality and recurrent infarction. A secondary endpoint of cerebrovascular events was assessed. Mean follow-up of the patients was 37 months. The results for each endpoint separately, including an analysis of vascular death, are provided in the following table:

RR=Relative risk; Risk reduction=(I - RR); CI=Confidence interval; MI=Myocardial infarction;
py=patient years
Event Warfarin


% Risk
Total Patient Years of Follow-up 2018 1944
Total Mortality Vascular Death 94 (4.7/100 py)
82 (4.1/100 py)
123 (6.3/100 py)
105 (5.4/100 py)
0.76 (0.60, 0.97)
0.78 (0.60, 1.02)
24 (p=0.030)
22 (p=0.068)
Recurrent MI 82 (4.1/100 py) 124 (6.4/100 py) 0.66 (0.51, 0.85) 34 (p=0.001)
Cerebrovascular Event 20 (1.0/100 py) 44  (2.3/100 py) 0.46 (0.28, 0.75) 54 (p=0.002)

Mechanical and Bioprosthetic Heart Valves:

In a prospective, randomized, open label, positive-controlled study (Mok et al, 1985) in 254 patients, the thromboembolic-free interval was found to be significantly greater in patients with mechanical prosthetic heart valves treated with Warfarin alone compared with dipyridamole-aspirin (p<0.005) and pentoxifylline-aspirin (p<0.05) treated patients. Rates of thromboembolic events in these groups were 2.2, 8.6, and 7.9/100 patient years, respectively. Major bleeding rates were 2.5, 0.0, and 0.9/100 patient years, respectively.

In a prospective, open label, clinical trial (Saour et al, 1990) comparing moderate (INR 2.65) vs. high intensity (INR 9.0) Warfarin therapies in 258 patients with mechanical prosthetic heart valves, thromboembolism occurred with similar frequency in the two groups (4.0 and 3.7 events/100 patient years, respectively). Major bleeding was more common in the high intensity group (2.1 events/100 patient years) vs. 0.95 events/100 patient years in the moderate intensity group.

In a randomized trial (Turpie et al, 1988) in 210 patients comparing two intensities of Warfarin therapy (INR 2.0 to 2.25 vs. INR 2.5 to 4.0) for a three month period following tissue heart valve replacement, thromboembolism occurred with similar frequency in the two groups (major embolic events 2.0% vs. 1.9%, respectively and minor embolic events 10.8% vs. 10.2%, respectively). Major bleeding complications were more frequent with the higher intensity (major hemorrhages 4.6%) vs. none in the lower intensity.


Warfarin sodium tablets are indicated for the prophylaxis and/or treatment of venous thrombosis and its extension, and pulmonary embolism.

Warfarin sodium tablets are indicated for the prophylaxis and/or treatment of the thromboembolic complications associated with atrial fibrillation and/or cardiac valve replacement.

Warfarin sodium tablets are indicated to reduce the risk of death, recurrent myocardial infarction, and thromboembolic events such as stroke or systemic embolization after myocardial infarction.


Anticoagulation is contraindicated in any localized or general physical condition or personal circumstance in which the hazard of hemorrhage might be greater than the potential clinical benefits of anticoagulation, such as:


Warfarin sodium is contraindicated in women who are or may become pregnant because the drug passes through the placental barrier and may cause fatal hemorrhage to the fetus in utero. Furthermore, there have been reports of birth malformations in children born to mothers who have been treated with Warfarin during pregnancy.

Embryopathy characterized by nasal hypoplasia with or without stippled epiphyses (chondrodysplasia punctata) has been reported in pregnant women exposed to Warfarin during the first trimester. Central nervous system abnormalities also have been reported, including dorsal midline dysplasia characterized by agenesis of the corpus callosum, Dandy-Walker malformation, and midline cerebellar atrophy. Ventral midline dysplasia, characterized by optic atrophy, and eye abnormalities have been observed. Mental retardation, blindness, and other central nervous system abnormalities have been reported in association with second and third trimester exposure. Although rare, teratogenic reports following in utero exposure to Warfarin include urinary tract anomalies such as single kidney, asplenia, anencephaly, spina bifida, cranial nerve palsy, hydrocephalus, cardiac defects and congenital heart disease, polydactyly, deformities of toes, diaphragmatic hernia, corneal leukoma, cleft palate, cleft lip, schizencephaly, and microcephaly.

Spontaneous abortion and still birth are known to occur and a higher risk of fetal mortality is associated with the use of Warfarin. Low birth weight and growth retardation have also been reported.

Women of childbearing potential who are candidates for anticoagulant therapy should be carefully evaluated and the indications critically reviewed with the patient. If the patient becomes pregnant while taking this drug, she should be apprised of the potential risks to the fetus, and the possibility of termination of the pregnancy should be discussed in light of those risks.

Hemorrhagic tendencies or blood dyscrasias.

Recent or contemplated surgery of: (1) central nervous system; (2) eye; (3) traumatic surgery resulting in large open surfaces.

Bleeding tendencies associated with active ulceration or overt bleeding of: (1) gastrointestinal, genitourinary or respiratory tracts; (2) cerebrovascular hemorrhage; (3) aneurysms-cerebral, dissecting aorta; (4) pericarditis and pericardial effusions; (5) bacterial endocarditis.

Threatened abortion, eclampsia and preeclampsia.

Inadequate laboratory facilities.

Unsupervised patients with senility, alcoholism, or psychosis or other lack of patient cooperation.

Spinal puncture and other diagnostic or therapeutic procedures with potential for uncontrollable bleeding.


Major regional, lumbar block anesthesia, malignant hypertension and known hypersensitivity to Warfarin or to any other components of this product.


The most serious risks associated with anticoagulant therapy with Warfarin sodium are hemorrhage in any tissue or organ and, less frequently (<0.1%), necrosis and/or gangrene of skin and other tissues. The risk of hemorrhage is related to the level of intensity and the duration of anticoagulant therapy. Hemorrhage and necrosis have in some cases been reported to result in death or permanent disability. Necrosis appears to be associated with local thrombosis and usually appears within a few days of the start of anticoagulant therapy. In severe cases of necrosis, treatment through debridement or amputation of the affected tissue, limb, breast or penis has been reported. Careful diagnosis is required to determine whether necrosis is caused by an underlying disease. Warfarin therapy should be discontinued when Warfarin is suspected to be the cause of developing necrosis and heparin therapy may be considered for anticoagulation. Although various treatments have been attempted, no treatment for necrosis has been considered uniformly effective. See below for information on predisposing conditions. These and other risks associated with anticoagulant therapy must be weighed against the risk of thrombosis or embolization in untreated cases.

It cannot be emphasized too strongly that treatment of each patient is a highly individualized matter. Warfarin sodium, a narrow therapeutic range (index) drug, may be affected by factors such as other drugs and dietary Vitamin K. Dosage should be controlled by periodic determinations of prothrombin time (PT)/International Normalized Ratio (INR) or other suitable coagulation tests. Determinations of whole blood clotting and bleeding times are not effective measures for control of therapy. Heparin prolongs the one-stage PT. When heparin and Warfarin sodium are administered concomitantly, refer below to Conversion From Heparin Therapy for recommendations.

Caution should be observed when Warfarin sodium is administered in any situation or in the presence of any predisposing condition where added risk of hemorrhage, necrosis, and/or gangrene is present.

Anticoagulation therapy with Warfarin sodium may enhance the release of atheromatous plaque emboli, thereby increasing the risk of complications from systemic cholesterol microembolization, including the “purple toes syndrome.” Discontinuation of Warfarin sodium therapy is recommended when such phenomena are observed.

Systemic atheroemboli and cholesterol microemboli can present with a variety of signs and symptoms including purple toes syndrome, livedo reticularis, rash, gangrene, abrupt and intense pain in the leg, foot, or toes, foot ulcers, myalgia, penile gangrene, abdominal pain, flank or back pain, hematuria, renal insufficiency, hypertension, cerebral ischemia, spinal cord infarction, pancreatitis, symptoms simulating polyarteritis, or any other sequelae of vascular compromise due to embolic occlusion. The most commonly involved visceral organs are the kidneys followed by the pancreas, spleen, and liver. Some cases have progressed to necrosis or death.

Purple toes syndrome is a complication of oral anticoagulation characterized by a dark, purplish or mottled color of the toes, usually occurring between 3 to 10 weeks, or later, after the initiation of therapy with Warfarin or related compounds. Major features of this syndrome include purple color of plantar surfaces and sides of the toes that blanches on moderate pressure and fades with elevation of the legs; pain and tenderness of the toes; waxing and waning of the color over time. While the purple toes syndrome is reported to be reversible, some cases progress to gangrene or necrosis which may require debridement of the affected area, or may lead to amputation.

Heparin-induced thrombocytopenia:

Warfarin sodium should be used with caution in patients with heparin-induced thrombocytopenia and deep venous thrombosis. Cases of venous limb ischemia, necrosis, and gangrene have occurred in patients with heparin-induced thrombocytopenia and deep venous thrombosis when heparin treatment was discontinued and Warfarin therapy was started or continued. In some patients sequelae have included amputation of the involved area and/or death (Warkentin et al, 1997).

A severe elevation (>50 seconds) in activated partial thromboplastin time (aPTT) with a PT/INR in the desired range has been identified as an indication of increased risk of postoperative hemorrhage.

The decision to administer anticoagulants in the following conditions must be based upon clinical judgment in which the risks of anticoagulant therapy are weighed against the benefits:


Based on very limited published data, Warfarin has not been detected in the breast milk of mothers treated with Warfarin. The same limited published data reports that breast-fed infants, whose mothers were treated with Warfarin, had prolonged prothrombin times, although not as prolonged as those of the mothers. The decision to breast-feed should be undertaken only after careful consideration of the available alternatives. Women who are breastfeeding and anticoagulated with Warfarin should be very carefully monitored so that recommended PT/INR values are not exceeded. It is prudent to perform coagulation tests and to evaluate Vitamin K status in infants at risk for bleeding tendencies before advising women taking Warfarin to breast-feed. Effects in premature infants have not been evaluated.

Severe to moderate hepatic or renal insufficiency.

Infectious diseases or disturbances of intestinal flora: sprue, antibiotic therapy.

Trauma which may result in internal bleeding.

Surgery or trauma resulting in large exposed raw surfaces.

Indwelling catheters.

Severe to moderate hypertension.

Known or suspected deficiency in protein C mediated anticoagulant response:

Hereditary or acquired deficiencies of protein C or its cofactor, protein S, have been associated with tissue necrosis following Warfarin administration. Not all patients with these conditions develop necrosis, and tissue necrosis occurs in patients without these deficiencies. Inherited resistance to activated protein C has been described in many patients with venous thromboembolic disorders but has not yet been evaluated as a risk factor for tissue necrosis. The risk associated with these conditions, both for recurrent thrombosis and for adverse reactions, is difficult to evaluate since it does not appear to be the same for everyone. Decisions about testing and therapy must be made on an individual basis. It has been reported that concomitant anticoagulation therapy with heparin for 5 to 7 days during initiation of therapy with Warfarin sodium may minimize the incidence of tissue necrosis. Warfarin therapy should be discontinued when Warfarin is suspected to be the cause of developing necrosis and heparin therapy may be considered for anticoagulation.


polycythemia vera, vasculitis, and severe diabetes.

Minor and severe allergic/hypersensitivity reactions and anaphylactic reactions have been reported.

In patients with acquired or inherited Warfarin resistance, decreased therapeutic responses to Warfarin sodium have been reported. Exaggerated therapeutic responses have been reported in other patients.

Patients with congestive heart failure may exhibit greater than expected PT/INR response to Warfarin sodium, thereby requiring more frequent laboratory monitoring, and reduced doses of Warfarin sodium.

Concomitant use of anticoagulants with streptokinase or urokinase is not recommended and may be hazardous. (Please note recommendations accompanying these preparations.)


Periodic determination of PT/INR or other suitable coagulation test is essential.

Numerous factors, alone or in combination, including travel, changes in diet, environment, physical state and medications, including botanicals, may influence response of the patient to anticoagulants. It is generally good practice to monitor the patient’s response with additional PT/INR determinations in the period immediately after discharge from the hospital, and whenever other medications, including botanicals, are initiated, discontinued or taken irregularly. The following factors are listed for reference; however, other factors may also affect the anticoagulant response.

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 altered 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.

The following factors, alone or in combination, may be responsible for INCREASED PT/INR response:

Endogenous Factors:

blood dyscrasias-see CONTRAINDICATIONS hepatic disorders:
infectious hepatitis
cancer jaundice
collagen vascular disease hyperthyroidism
congestive heart failure poor nutritional state
diarrhea steatorrhea
elevated temperature vitamin K deficiency

Exogenous Factors:

Potential drug interactions with Warfarin sodium are listed below by drug class and by specific drugs.

*Increased and decreased PT/INR responses have been reported.

Classes of Drugs

5-lipoxygenase Inhibitor Antiparasitic/Antimicrobials HMG-CoA Reductase
Adrenergic Stimulants, Central Antiplatelet Drugs/Effects Inhibitors*
Alcohol Abuse Reduction Preparations Antithyroid Drugs* Leukotriene Receptor Antagonist
Analgesics Beta-Adrenergic Blockers Monoamine Oxidase Inhibitors
Anesthetics, Inhalation Cholelitholytic Agents Narcotics, prolonged
Antiandrogen Diabetes Agents, Oral Nonsteroidal Anti-Inflammatory Agents
Antiarrhythmics* Diuretics* Psychostimulants
Antibiotics* Fungal Medications, Intravaginal, Systemic* Pyrazolones
Aminoglycosides (oral) Gastric Acidity and Peptic Ulcer Agents* Salicylates
Cephalosporins, parenteral Gastrointestinal Selective Serotonin Reuptake Inhibitors
Macrolides Prokinetic Agents Steroids, Adrenocortical*
Miscellaneous Ulcerative Colitis Agents Steroids, Anabolic (17-Alkyl Testosterone Derivatives)
Penicillins, intravenous, high dose Gout Treatment Agents Thrombolytics
Quinolones (fluoroquinolones) Hemorrheologic Agents Thyroid Drugs
Sulfonamides, long acting Hepatotoxic Drugs Tuberculosis Agents*
Tetracyclines Hyperglycemic Agents Uricosuric Agents
Anticoagulants Hypertensive Emergency Agents Vaccines
Anticonvulsants* Hypnotics* Vitamins*
Antidepressants* Hypolipidemics*
Antimalarial Agents Bile Acid-Binding Resins*
Antineoplastics* Fibric Acid Derivatives
Specific Drugs Reported
acetaminophen cimetidine flutamide methyldopa pentoxifylline sulfisoxazole
alcohol* ciprofloxacin fluvastatin methylphenidate phenylbuta-
allopurinol cisapride fluvoxamine methylsalicylate phenytoin* tamoxifen
aminosalicylic acid clarithromy-
gemflbrozil ointment (topical) piperacillin tetracycline
amiodarone HCl clofibrate glucagon metronidazole, piroxicam thyroid
aspirin cyclopho-
halothane miconazole intravaginal, systemic pravastatin* ticarcillin
atorvastatin* danazol heparin prednisone* ticlopidine
azithromycin dextran ibuprofen propafenone tissue plasminogen activator (t-PA)
capecitabine dextrothyrox-ine ifosfamide moricizine hydrochloride* propoxyphene
cefamandole diazoxide indomethacin propranolol tolbutamide
cefazolin diclofenac influenza virus vaccine nalidixic acid propylthioura-
cefoperazone dicumarol naproxen quinidine trimethoprim/
cefotetan diflunisal itraconazole neomycin quinine
cefoxitin disulfiram ketoprofen norfloxacin ranitidine*
ceftriaxone doxycycline ketorolac ofloxacin rofecoxib urokinase
celecoxib erythromycin levamisole olsalazine sertraline valproate
cerivastatin ethacrynic acid levofloxacin omeprazole simvastatin vitamin E
chenodiol fenofibrate levothyroxine oxaprozin stanozolol Warfarin sodium overdosage
chloramphenicol fenoprofen liothyronine oxymetholone streptokinase
chloral hydrate* fluconazole lovastatin paroxetine sulfamethizole zafirlukast
chlorpropamide fluorouracil mefenamic acid penicillin G, intravenous sulfamethoxa-zole zileuton
cholestyramine* fluoxetine methimazole* sulfinpyrazone
also: other medications affecting blood elements which may modify hemostasis
dietary deficiencies
prolonged hot weather
unreliable PT/INR determinations

The following factors, alone or in combination, may be responsible for DECREASED PT/INR response:

Endogenous Factors:

edema hereditary coumarin resistance hyperlipemia hypothyroidism nephrotic syndrome

Exogenous Factors:

Potential drug interactions with Warfarin sodium are listed below by drug class and by specific drugs.

* Increased and decreased PT/INR responses have been reported.
Classes of Drugs
Adrenal Cortical Steroid Inhibitors Antipsychotic Medications Hypolipidemics* Steroids, Adrenocortical*
Antacids Antithyroid Drugs* Bile Acid-Binding Resins* Tuberculosis Agents*
Antianxiety Agents Barbiturates HMG-CoA Reductase Inhibitors* Vitamins*
Antiarrhythmics* Diuretics*
Antibiotics* Enteral Nutritional Supplements Immunosuppressives
Anticonvulsants* Fungal Medications, Systemic* Oral Contraceptives, Estrogen Containing
Antidepressants* Gastric Acidity and Peptic Ulcer Agents*
Antihistamines Selective Estrogen Receptor Modulators
Antineoplastics* Hypnotics*
Specific Drugs Reported
alcohol* chlordiazepoxide glutethimide pentobarbital rifampin
aminoglutethimide chlorthalidone griseofulvin phenobarbital secobarbital
amobarbital cholestyramine* haloperidol phenytoin* spironolactone
atorvastatin* clozapine meprobamate pravastatin* sucralfate
azathioprine corticotropin 6-mercaptopurine prednisone* trazodone
butabarbital cortisone methimazole* primidone vitamin C (high dose)
butalbital cyclophosphamide* moricizine hydrochloride* propylthiouracil* vitamin K
carbamazepine dicloxacillin nafcillin raloxifene Warfarin sodium underdosage
chloral hydrate* ethchlorvynol paraldehyde ranitidine*
also: diet high in vitamin K
unreliable PT/INR determinations

Because a patient may be exposed to a combination of the above factors, the net effect of Warfarin sodium on PT/INR response may be unpredictable. More frequent PT/INR monitoring is therefore advisable. Medications of unknown interaction with coumarins are best regarded with caution. When these medications are started or stopped, more frequent PT/INR monitoring is advisable.

It has been reported that concomitant administration of Warfarin and ticlopidine may be associated with cholestatic hepatitis.

Botanical (Herbal) Medicines:

Caution should be exercised when botanical medicines (botanicals) are taken concomitantly with Warfarin sodium. Few adequate, well-controlled studies exist evaluating the potential for metabolic and/or pharmacologic interactions between botanicals and Warfarin sodium. Due to a lack of manufacturing standardization with botanical medicinal preparations, the amount of active ingredients may vary. This could further confound the ability to assess potential interactions and effects on anticoagulation. It is good practice to monitor the patient’s response with additional PT/INR determinations when initiating or discontinuing botanicals.

Specific botanicals reported to affect Warfarin sodium therapy include the following:

  • Bromelains, danshen, dong quai (Angelica sinensis), garlic, Ginkgo biloba and ginseng are associated most often with an INCREASE in the effects of Warfarin sodium.
  • Coenzyme Q10 (ubidecarenone) and St. John’s wort are associated most often with a DECREASE in the effects of Warfarin sodium.

Some botanicals may cause bleeding events when taken alone (e.g., garlic and Ginkgo biloba) and may have anticoagulant, antiplatelet, and/or fibrinolytic properties. These effects would be expected to be additive to the anticoagulant effects of Warfarin sodium. Conversely, other botanicals may have coagulant properties when taken alone or may decrease the effects of Warfarin sodium. Some botanicals that may affect coagulation are listed below for reference; however, this list should not be considered all-inclusive. Many botanicals have several common names and scientific names. The most widely recognized common botanical names are listed.

Contains coumarins and salicylate.
Contains coumarins and has antiplatelet properties.
Contains coumarins and has fibrinolytic properties.
Contains salicylate and has coagulant properties.
Has antiplatelet and fibrinolytic properties.
Botanicals that contain coumarins with potential anticoagulant effects:
Alfalfa Celery Parsley
Angelica (Dong Quai) Chamomile Passion Flower
Aniseed (German and Roman) Prickly Ash (Northern)
Arnica Dandelion Quassia
Asa Foetida Fenugreek Red Clover
Bogbean* Horse Chestnut Sweet Clover
Boldo Horseradish Sweet Woodruff
Buchu Licorice  Tonka Beans
Capsicum Meadowsweet*  Wild Carrot
Cassia  Nettle Wild Lettuce
Miscellaneous botanicals with anticoagulant properties:
Bladder Wrack (Fucus) Pau d’arco
Botanicals that contain salicylate and/or have antiplatelet properties:
Agrimony§  Dandelion Meadowsweet* 
Aloe Gel Feverfew Onion 
Aspen Garlic  Policosanol
Black Cohosh German Sarsaparilla Poplar
Black Haw Ginger Senega
Bogbean* Ginkgo Biloba Tamarind
Cassia  Ginseng (Panax) Willow
Clove Licorice  Wintergreen
Botanicals with fibrinolytic properties:
Bromelains Garlic  Inositol Nicotinate
Capsicum Ginseng (Panax) Onion 
Botanicals with coagulant properties:
Agrimony§ Mistletoe 
Goldenseal Yarrow

Effect on Other Drugs:

Coumarins may also affect the action of other drugs. Hypoglycemic agents (chlorpropamide and tolbutamide) and anticonvulsants (phenytoin and phenobarbital) may accumulate in the body as a result of interference with either their metabolism or excretion.

Special Risk Patients:

Warfarin sodium is a narrow therapeutic range (index) drug, and caution should be observed when Warfarin sodium is administered to certain patients such as the elderly or debilitated or when administered in any situation or physical condition where added risk of hemorrhage is present.

Intramuscular (I.M.) injections of concomitant medications should be confined to the upper extremities which permits easy access for manual compression, inspections for bleeding and use of pressure bandages.

Caution should be observed when Warfarin sodium is administered concomitantly with nonsteroidal anti-inflammatory drugs (NSAIDs), including aspirin, to be certain that no change in anticoagulation dosage is required. In addition to specific drug interactions that might affect PT/INR, NSAIDs, including aspirin, can inhibit platelet aggregation, and can cause gastrointestinal bleeding, peptic ulceration and/or perforation.

Acquired or inherited Warfarin resistance should be suspected if large daily doses of Warfarin sodium are required to maintain a patient’s PT/INR within a normal therapeutic range.

Information for Patients:

The objective of anticoagulant therapy is to decrease the clotting ability of the blood so that thrombosis is prevented, while avoiding spontaneous bleeding. Effective therapeutic levels with minimal complications are in part dependent upon cooperative and well-instructed patients who communicate effectively with their physician. Patients should be advised: Strict adherence to prescribed dosage schedule is necessary. Do not take or discontinue any other medication, including salicylates (e.g., aspirin and topical analgesics), other over-the-counter medications, and botanical (herbal) products (e.g., bromelains, coenzyme Q10, danshen, dong quai, garlic, Ginkgo biloba, ginseng, and St. John’s wort) except on advice of the physician. Avoid alcohol consumption. Do not take Warfarin sodium during pregnancy and do not become pregnant while taking it (see CONTRAINDICATIONS). Avoid any activity or sport that may result in traumatic injury. Prothrombin time tests and regular visits to physician or clinic are needed to monitor therapy. Carry identification stating that Warfarin sodium is being taken. If the prescribed dose of Warfarin sodium is forgotten, notify the physician immediately. Take the dose as soon as possible on the same day but do not take a double dose of Warfarin sodium the next day to make up for missed doses. The amount of vitamin K in food may affect therapy with Warfarin sodium. Eat a normal, balanced diet maintaining a consistent amount of vitamin K. Avoid drastic changes in dietary habits, such as eating large amounts of green leafy vegetables. Contact physician to report any illness, such as diarrhea, infection or fever. Notify physician immediately if any unusual bleeding or symptoms occur. Signs and symptoms of bleeding include: pain, swelling or discomfort, prolonged bleeding from cuts, increased menstrual flow or vaginal bleeding, nosebleeds, bleeding of gums from brushing, unusual bleeding or bruising, red or dark brown urine, red or tar black stools, headache, dizziness, or weakness. If therapy with Warfarin sodium is discontinued, pat

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