Coadministration has not been studied and is not recommended. Primidone is metabolised by CYP3A4 to the active metabolite phenobarbital. Phenobarbital induces P-gp and is expected to decrease tenofovir alafenamide exposure. However, data from a study with rifampicin suggest that use of tenofovir alafenamide 25 mg once daily with primidone 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.