Coadministration has not been studied. Dexamethasone is metabolized by CYP3A4 and elvitegravir/cobicistat may increase dexamethasone concentrations due to inhibition of CYP3A4. A dose reduction of the glucocorticoid may be necessary with monitoring for symptoms of Cushing's syndrome. Dexamethasone is a dose-dependent inducer of CYP3A4 and is a moderate CYP3A4 inducer at doses above 16 mg. Chronic or high doses of dexamethasone, a CYP3A inducer, may significantly decrease elvitegravir and cobicistat plasma concentrations, which may result in loss of therapeutic effect and development of resistance. Use with caution. Alternative corticosteroids should be considered. No interaction is expected with emtricitabine, however, dexamethasone induces intestinal P-glycoprotein and could reduce the absorption of tenofovir alafenamide, although to a moderate extent. No a priori dose adjustment of tenofovir alafenamide is needed as coadministration with rifampicin (a stronger inducer) led to lower intracellular tenofovir diphosphate concentrations but which were still higher than those achieved with tenofovir disoproxil fumarate.