virazole and Coumadin Interactions

Interactions between virazole(ribavirin) and Coumadin (warfarin)

warfarin and ribavirin (Moderate Drug-Drug)

MONITOR: Ribavirin may decrease the hypoprothrombinemic effect of warfarin. The mechanism of interaction is unknown. In an isolated case report, a 61-year-old man previously stabilized on 45 mg/week of warfarin required an approximately 40% increase in the anticoagulant dosage over a period of one month following addition of interferon alfa-2b and ribavirin therapy for hepatitis C. After discontinuation of interferon-ribavirin therapy a year later, the anticoagulant dosage decreased to 47.5 mg/week over a three-week period. A subsequent rechallenge with ribavirin appeared to confirm the interaction. The effect of ribavirin on other oral anticoagulants is unknown.

MANAGEMENT: Patients treated with warfarin should be closely monitored during concomitant therapy with ribavirin. The INR should be checked frequently (e.g., weekly) during the 4 weeks following initiation or discontinuation of ribavirin therapy in patients who are stabilized on their warfarin regimen, and the warfarin dosage adjusted accordingly. The same precaution may be applicable during therapy with other oral anticoagulants, although clinical data are lacking. Patients should be advised to promptly report any signs of bleeding to their physician, including pain, swelling, headache, dizziness, weakness, prolonged bleeding from cuts, increased menstrual flow, vaginal bleeding, nosebleeds, bleeding of gums from brushing, unusual bleeding or bruising, red or brown urine, or red or black stools.