Coadministration of darunavir/cobicistat and rifabutin has not been studied and is not recommended as it may significantly decrease cobicistat plasma concentrations and consequently those of darunavir being boosted. Coadministration of elvitegravir/cobicistat (150 mg/150 mg once daily) and rifabutin (150 mg every other day) decreased cobicistat Ctrough by 66%. Rifabutin exposure was similar to values obtained alone, but 25-O-desacetylrifabutin exposures were increased (AUC, Cmax and Cmin were increased by 525%, 384% and 394%). The European SPC for darunavir/cobicistat recommends that if the combination is needed, to use rifabutin 150 mg 3 times per week on set days (e.g. Monday-Wednesday-Friday); the US prescribing information for darunavir/cobicistat recommends rifabutin 150 mg every other day. Both labels recommend increased monitoring for rifabutin associated adverse reactions including neutropenia and uveitis due to increased rifabutin exposure. Further dose reduction of rifabutin has not been studied and a twice weekly dose of 150 mg may not provide an optimal exposure to rifabutin leading to a risk of rifamycin resistance and a treatment failure.