Coadministration with emtricitabine/tenofovir-DF has not been studied. Coadministration of tenofovir-DF (300 mg once daily) and atazanavir/ritonavir (300/100 mg once daily) was studied in 12 subjects. Atazanavir AUC, Cmax and Cmin decreased by 25%, 28% and 26%, respectively. Tenofovir AUC, Cmax and Cmin increased by 37%, 34% and 29%, respectively. No dose adjustment is recommended. The increased exposure of tenofovir could potentiate tenofovir associated adverse events, including renal disorders. Renal function should be closely monitored. In HIV-1 infected patients with renal risk factors, the co-administration of tenofovir-DF with a boosted protease inhibitor should be carefully evaluated. Cobicistat has been shown to decrease estimated creatinine clearance due to inhibition of tubular secretion of creatinine without affecting actual renal glomerular function. Atazanavir/cobicistat should not be coadministered with emtricitabine or tenofovir-DF in patients with creatinine clearance less than 70 ml/min.