warfarin (Coumadin, Jantoven)

  • Medical Author:
    Omudhome Ogbru, PharmD

    Dr. Ogbru received his Doctorate in Pharmacy from the University of the Pacific School of Pharmacy in 1995. He completed a Pharmacy Practice Residency at the University of Arizona/University Medical Center in 1996. He was a Professor of Pharmacy Practice and a Regional Clerkship Coordinator for the University of the Pacific School of Pharmacy from 1996-99.

  • Medical Editor: Jay W. Marks, MD
    Jay W. Marks, MD

    Jay W. Marks, MD

    Jay W. Marks, MD, is a board-certified internist and gastroenterologist. He graduated from Yale University School of Medicine and trained in internal medicine and gastroenterology at UCLA/Cedars-Sinai Medical Center in Los Angeles.

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PREPARATIONS: Tablets: 1, 2, 2.5, 3, 4, 5, 6, 5, 7.5 and 10 mg. Powder for Injection: 5 mg/vial

STORAGE: Warfarin should be stored at room temperature, 59 F to 86 F (15 C to 30 C), in tight, light resistant container.

DOSING: Warfarin may be taken with or without food. Since warfarin is metabolized (inactivated) by the liver and then excreted by the kidneys, dosages need to be lowered in patients with liver and kidney dysfunction. Frequent blood tests (INR test) are performed to measure the effect of warfarin and to adjust dosing. There are published INR ranges for the various uses of warfarin. Treatment usually is started at 2 to 5 mg once daily and the dose is adjusted based in INR tests. Patients typically require 2 to 10 mg of warfarin daily.

DRUG INTERACTIONS: Many drugs, both prescription and nonprescription (OTC), can affect the anticoagulant action of warfarin or increase the risk of bleeding. Patients on warfarin should regularly consult their doctor before instituting any medications on their own. It also is advisable for patients on warfarin to carry identification such as bracelets to alert other health professionals to the presence of anticoagulation. Following are a few examples of drugs that interact with warfarin. Drugs the increase the effect of warfarin by reducing the breakdown of warfarin include amiodarone (Cordarone), trimethoprim/sulfamethoxazole (Bactrim), fluconazole (Diflucan), itraconazole (Sporanox), fluvastatin, fluvoxamine, metronidazole miconazole, voriconazole (Vfend), zafirlukast (Accolate), ciprofloxacin (Cipro), cimetidine, atorvastatin (Lipitor), clarithromycin (Biaxin), fluoxetine (Prozac), indinavir (Crixivan), and ritonavir (Norvir).

Drugs that may reduce the effect of warfarin by increasing its breakdown include St. John's wort, carbamazepine (Tegretol, Tegretol XR, Equetro, Carbatrol), rifampin, bosentan (Tracleer), and prednisone.

Bleeding is increased by other anticoagulants such as heparin, argatroban (Acova), dabigatran (Pradaxa), and others; antiplatelet drugs such as aspirin and other nonsteroidal anti-inflammatory drugs (for example, ibuprofen [Motrin], naproxen [Alleve]), clopidogrel (Plavix), and prasugrel (Effient); serotonin reuptake inhibitors (SSRIs) such as fluoxetine (Prozac) and paroxetine (Paxil). Garlic and ginkgo also increase the risk of bleeding because they cause bleeding when taken alone.

Foods with high vitamin K content (for example, green leafy vegetables) reduce the effect of warfarin. Maintenance of a consistent intake of vitamin K containing foods is important to avoid fluctuations in the effect of warfarin.

PREGNANCY: Warfarin should be avoided by pregnant women or women who may become pregnant. Birth defects and fetal bleeding have been reported.

NURSING MOTHERS: Available evidence suggests that warfarin is not secreted in breast milk.

Medically reviewed by Eni Williams, PharmD

Reference: FDA Prescribing Information

Medically Reviewed by a Doctor on 10/16/2015

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