Coadministration has not been studied. Warfarin is a mixture of enantiomers which are metabolised by different cytochromes. R-warfarin is primarily metabolised by CYP1A2 and 3A4. S-warfarin (more potent) is metabolised by CYP2C9. The net effect is difficult to predict as elvitegravir is a modest inducer of CYP2C9 and cobicistat is a strong inhibitor of CYP3A4. A case report of coadministration of warfarin with Stribild (elvitegravir, cobicistat, emtricitabine, tenofovir-DF) described decreased warfarin exposure which required a 60% increase in warfarin dose, suggesting that CYP2C9 induction by elvitegravir has a stronger effect on warfarin metabolism than CYP3A inhibition by cobicistat. Monitor INR during coadministration and for the first weeks after stopping treatment with Genvoya. Emtricitabine and tenofovir alafenamide do not interact with this metabolic pathway.