Coadministration has not been studied but based on metabolism and clearance a clinically significant interaction is unlikely as elvitegravir is metabolised by CYP3A4 which is not affected by sofosbuvir. Sofosbuvir is a prodrug and formation of its active metabolite is unlikely to be affected by comedications. In addition, coadministration of emtricitabine/tenofovir-DF (with efavirenz) and sofosbuvir (400 mg single dose) was studied in 16 subjects. Sofosbuvir Cmax and AUC decreased by 19% and 6%, respectively, with GS-331007 Cmax and AUC decreasing by 23% and 16%, respectively. Tenofovir Cmax increased by 25%, but AUC and Cmin decreased by 2% and 1%, respectively. No dose adjustment of sofosbuvir or tenofovir-DF is required.