Coadministration has not been studied and is not recommended. Phenytoin is mainly metabolised by CYP2C9 and to a lesser extent by CYP2C19. Emtricitabine and tenofovir (derived from tenofovir alafenamide) are eliminated renally. No interaction is expected with emtricitabine. Phenytoin, a P-gp inducer, may decrease tenofovir alafenamide plasma concentrations. However, data from a study with rifampicin suggest that use of tenofovir alafenamide 25 mg once daily with phenytoin may be acceptable. Coadministration of emtricitabine/tenofovir alafenamide (200/25 mg once daily) and the strong inducer rifampicin (600 mg once daily) decreased plasma exposure of tenofovir alafenamide and tenofovir by ~55%. Intracellular tenofovir-DP AUC decreased by 36%, however, intracellular tenofovir-DP exposure was 4.2-fold higher than that achieved with standard dose tenofovir-DF alone (300 mg once daily). No interaction is expected with emtricitabine.