Based on theoretical considerations darunavir/cobicistat may alter warfarin plasma concentrations. It is recommended that the international normalised ratio (INR) be monitored when warfarin is co-administered with darunavir/cobicistat.

The effect of coadministration on warfarin concentrations is unknown. Monitor the international normalized ratio (INR) when co-administering with warfarin 

A case reports describes a change in INR (and subsequent warfarin dose modifications) in a patient who switched from a darunavir/ritonavir containing regimen to a darunavir/cobicistat containing regimen. The patient’s INR increased over several months despite decreases in warfarin dose over the same period of time. A dose reduction of warfarin to 4 mg daily (from 10 mg daily) was required in order to return the INR to baseline.