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All about: Warfarin Sodium

Big Image Pronouncation: (WORE-fuh-rin)
Class: Anticoagulants

Trade Names:
Coumadin
- Tablets 1 mg
- Tablets 2 mg
- Tablets 2.5 mg
- Tablets 3 mg
- Tablets 4 mg
- Tablets 5 mg
- Tablets 6 mg
- Tablets 7.5 mg
- Tablets 10 mg
- Powder for Injection, lyophilized 2 mg

Apo-Warfarin (Canada)
Gen-Warfarin (Canada)
Taro-Warfarin (Canada)

Mechanism of Action

Pharmacology

Interferes with hepatic synthesis of vitamin K-dependent clotting factors, resulting in in-vivo depletion of clotting factors II, VII, IX, and X.

Pharmacokinetics

Absorption

It is completely absorbed. The T max is 4 hr.

Distribution

Vd is about 0.14 L/kg. It is about 99% bound to plasma protein.

Metabolism

The elimination of warfarin is almost entirely by metabolism. It is metabolized by cytochrome P450 to inactive hydroxylated metabolites (predominant route) and by reductases to reduced metabolites (warfarin alcohols).

Elimination

Metabolites are principally excreted into urine and to a lesser extent into the bile. The t ½ after a single dose is about 1 wk; however, the effective t ½ ranges from 20 to 60 hr. The clearance of R-warfarin is 50% that of S-warfarin. The t ½ of R-warfarin is about 37 to 89 hr and it is about 21 to 43 hr in S-warfarin. About 92% of orally administered doses are recovered in urine.

Special Populations

Hepatic Function Impairment

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

Elderly

Patients at least 60 yr appear to exhibit greater than expected prothrombin time (PT)/international normalized ratio (INR) response to warfarin. As a patient's age increases, a lower dose is usually required to produce a therapeutic level of anticoagulation.

Indications and Usage

Prophylaxis and treatment of venous thrombosis and its extension; prophylaxis and treatment of atrial fibrillation with embolization; prophylaxis and treatment of pulmonary embolism; adjunct in prophylaxis of systemic embolism after MI.

Unlabeled Uses

Prevention of recurrent transient ischemic attacks and reduction of risk of recurrent MI; adjunctive treatment of small cell carcinoma of lung.

Contraindications

Pregnancy; hemorrhagic tendencies; hemophilia; thrombocytopenic purpura; leukemia; recent or contemplated surgery of eye or CNS, major regional lumbar block anesthesia, or surgery resulting in large, open surfaces; patients bleeding from GI, respiratory, or GU tract; threatened abortion; aneurysm; ascorbic acid deficiency; history of bleeding diathesis; prostatectomy; continuous tube drainage of small intestine; polyarthritis; diverticulitis; emaciation; malnutrition; cerebrovascular hemorrhage; eclampsia and preeclampsia; blood dyscrasias; severe uncontrolled or malignant hypertension; severe renal or hepatic disease; pericarditis and pericardial effusion; subacute bacterial endocarditis; visceral carcinoma; following spinal puncture and other diagnostic or therapeutic procedures (eg, IUD insertion) with potential for uncontrollable bleeding; history of warfarin-induced necrosis.

Dosage and Administration

Adults

PO 2 to 5 mg/day initially for 2 to 4 days; adjust daily dose according to PT or INR determinations. Usual maintenance dose is PO 2 to 10 mg/day.

Elderly

Lower dosages are recommended.

Adults

IV Provides an alternative administration route for patients who cannot receive oral drugs. The IV dosages would be the same as those that would be used orally. Administer as a slow bolus injection over 1 to 2 min in a peripheral vein.

General Advice

  • Do not give large loading dose.
  • Do not switch brands.
  • Administer at same time each day.
  • For IV use, give a slow bolus injection over 1 to 2 min in a peripheral vein.
  • Reconstitute IV vial with 2.7 mL of Sterile Water for Injection.
  • After reconstitution, solution is stable for 4 hr at room temperature.
  • Check solution for particle matter or discoloration immediately before use. If either is present, do not use.
  • Discard unused solution.

Storage/Stability

Store at room temperature in tight, light-resistant container.

Drug Interactions

Aminoglutethimide, azathioprine, barbiturates, carbamazepine, cholestyramine, ethchlorvynol, ginseng, glutethimide, griseofulvin, mercaptopurine, rifabutin, rifampin, St. John's wort, trazodone, ubiquinone, and vitamin K

Decreased anticoagulant effect of warfarin.

Androgens, amiodarone, cefamandole, cefazolin, cefoperazone, cefotetan, cefoxitin, ceftriaxone, chloramphenicol, cimetidine, clofibrate, danshen, dextrothyroxine, disulfiram, dong quai, erythromycin, fluconazole, glucagon, methimazole, metronidazole, miconazole, moxalactam, nalidixic acid, NSAIDs, phenylbutazone, propylthiouracil, quinidine, quinine, salicylates, sulfinpyrazone, sulfonamides, thyroid hormones, tricyclic antidepressants, and vitamin E

Increased anticoagulant effect of warfarin.

Hydantoins

Serum hydantoin concentration may be elevated, increasing risk of toxicity.

Laboratory Test Interactions

Oral anticoagulants may cause red-orange discoloration of alkaline urine, interfering with some laboratory tests.

Adverse Reactions

Dermatologic

Skin necrosis; gangrene; exfoliative dermatitis; urticaria; alopecia.

EENT

Mouth ulcers.

GI

Nausea; vomiting; diarrhea; paralytic ileus; intestinal obstruction; anorexia; abdominal cramps.

Genitourinary

Red-orange urine.

Hematologic

Hemorrhage; leukopenia.

Hepatic

Hepatotoxicity; cholestatic jaundice.

Miscellaneous

Fever; cholesterol microembolization (purple toe syndrome); hypersensitivity.

Precautions

Pregnancy

Category X .

Lactation

Excreted in breast milk.

Children

Safety and efficacy not established in children under 18 yr.

Elderly

May be more sensitive to effects.

Hypersensitivity

Reactions range from mild to life-threatening. Symptoms may be dermatologic (eg, erythema, eczematous rash, exfoliative dermatitis, exudative erythema multiforme, alopecia), hematologic (eg, eosinophilia, leukopenia, thrombocytopenia), renal, (eg, nephropathy, nephritis, oliguria), GI (eg, enanthema, severe stomatitis), or hepatic (eg, mixed hepatocellular damage, cholestasis, jaundice). If signs or symptoms occur, discontinue therapy and notify health care provider.

Hepatic Function

Use cautiously.

Special Risk Patients

There is increased risk associated with using warfarin in patients with trauma, infection, renal insufficiency, dietary insufficiency, uncontrolled hypertension, polycythemia vera, vasculitis, indwelling catheters. Evaluate benefits of therapy vs risks.

Adrenal hemorrhage

Discontinue therapy if patient develops signs and symptoms of adrenal insufficiency.

Hemorrhage/Necrosis

Most serious risks of therapy; may result in death.

Monitoring/PT

Individualize treatment based on PT or INR.

Protein C deficiency

Hereditary, familial, or clinical protein C deficiency has been associated with necrosis following warfarin therapy. If warfarin is suspected cause of necrosis, discontinue administration.

Purple toe syndrome

Systemic cholesterol microembolization from release of atheromatous plaque emboli. Discontinue therapy.

Surgical/Dental procedures

Adjust dose to maintain PT or INR at low end of therapeutic range for patients who must be anticoagulated during dental or surgical procedures.

Overdosage

Symptoms

Hematuria, excessive menstrual bleeding, melena, petechiae, oozing from superficial injuries.

Patient Information

  • Caution patient that this medication must not be taken during pregnancy or when pregnancy is possible. Advise patient to use reliable form of birth control while taking this drug.
  • Advise patient not to change dose unless advised by health care provider.
  • Advise patient not to drastically change diet or consume alcohol.
  • Advise patient not to change brands of medicine.
  • Advise patient to limit intake of vitamin K-rich foods, including avocados, bananas, broccoli, dried fruits, grapefruit, lima beans, nuts, oranges, peaches, potatoes, sunflower seeds, spinach, and tomatoes.
  • Instruct patient to report any GI upset, pink or red discoloration of urine, red or tar-black stools or diarrhea, rash, yellowish tint of skin or eyes, unusual bleeding (eg, heavier than normal menstrual flow), or bruising.
  • Caution patient not to take aspirin or other salicylates without consulting health care provider.
  • Instruct patient in safety practices: Use of soft toothbrush, electric razor, night lights, and avoidance of activities that could result in bruising or bleeding.
  • Tell patient not to take any OTC or prescription medications without consulting health care provider.
  • Remind patient to wear Medi-Alert identification bracelet.

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