(See Summary)

Coadministration with warfarin (5 mg single dose) and Viekirax + dasabuvir increased R-warfarin Cmax by 5%, but decreased AUC and Cmin by 12% and 6%; the Cmax, AUC and Cmin of S-warfarin decreased by 4%, 12% and 5%, respectively. Ombitasvir Cmax, AUC and Cmin decreased by 6%, 4% and 2%. Paritaprevir Cmax and Cmin decreased by 2% and 4%, but AUC increased by 7%. The magnitude of interaction with Viekirax alone was similar to that observed with Viekirax + dasabuvir. While no dose adjustment is necessary for warfarin, appropriate monitoring of international normalised ratio (INR) is recommended.
Viekirax Summary of Product Characteristics, AbbVie Ltd, January 2015.

A case report describes a patient's international normalized ratio (INR) becoming subtherapeutic two weeks after starting ombitasvir/paritaprevir/ritonavir + dasabuvir with ribavirin, Eleven weeks into therapy and following a 125% total increase in the weekly warfarin dose, therapeutic INR was achieved. Thirteen days after DAA therapy was completed and discontinued, the patient's INR became critically supratherapeutic. The INR should be monitored closely upon initiation and discontinuation of therapy.
Paritaprevir/ritonavir/ombitasvir+dasabuvir plus ribavirin therapy and inhibition of the anticoagulant effect of warfarin: a case report. Puglisi GM, Smith SM, Jankovich RD et al. J Clin Pharm Ther. 2016, [Epub ahead of print].