Ribavirin Warfarin

Inhibition of the effect of warfarin by ribavirin has been described. 

In a 61-year-old man, who was taking long-term warfarin after a heart valve replacement, the dose of warfarin had to be increased by 40% after the introduction of ribavirin for chronic hepatitis C the inhibition of the effect of warfarin was reproduced on rechallenge (272). 

Schulman S. Inhibition of warfarin activity by ribavirin. Ann Pharmacother 2002 36(1) 72. ... 

An interaction of warfarin with ribavirin has been reported (26). 

In a 61-year-old white man with chronic hepatitis C, who took interferon plus ribavirin, the dosage of warfarin had to be increased by about 40% (from 45 to 63 mg/week) in order to maintain the desired degree of anticoagulation. This effect was reproduced on rechallenge with ribavirin. 

The mechanism of this supposed interaction is not known. For example, ribavirin is cleared by intracellular phosphorylation and its metabolites by the kidneys, warfarin by cytochrome P450 isozymes in the liver warfarin is highly protein bound, ribavirin is not. However, an effect on warfarin absorption or its action on clotting factor synthesis is possible. 

These reports seem to be the only ones to describe this interaction so the interaction is not yet established. However it would seem prudent to monitor the effects if interferon is given to patients taking coumarins, reducing the dosage if necessary. For a case of decreased warfarin effects in a patient given interferon alfa-2b and ribavirin, see Coumarins + Ribavirin , p.447. 

In a single patient, ribavirin appeared to decrease the effect of warfarin requiring a 40% increase in warfarin dose. 

Unknown. The few cases with interferon , (p.422) have suggested that this may increase the effect of warfarin. In this case, ribavirin seems to have decreased the effect of warfarin, and overridden any effect of interferon. 

An apparent reaction between telaprevir and warfarin was reported in a patient xmdergoing treatment for chronic hepatitis C infection [15 ]. A 45-year-old Hispanic man had been on warfarin treatment with a stable INR for 8 months. He skipped five daily warfarin doses upon the initiation of triple therapy for chronic hepatitis C virus infection consisting of peginterferon alfa-2, ribavirin, and telaprevir to reattain a target INR, the warfarin dose had to be increased to 50% above the baseline dose. The warfarin dosing requirement declined only after the patient finished the prescribed telaprevir course. 

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