Coadministration with emtricitabine/tenofovir-DF has not been studied. Based on metabolism and clearance a clinically significant interaction with emtricitabine is unlikely. Coadministration of emtricitabine and efavirenz in the presence of didanosine was studied in 9 treatment naive, HIV+ patients. When compared to historical data for each drug alone, the pharmacokinetics of emtricitabine were not affected, didanosine AUC and Cmax were higher than expected and efavirenz Cmax, Cmin and AUC were lower than expected. However, efavirenz concentrations may have been underestimated as dosing was delayed by 12 hours for ease of sampling. Coadministration of tenofovir-DF and efavirenz decreased efavirenz AUC and Cmax both by 4%; tenofovir AUC decreased by 1% and Cmax increased by 7%. No dosage adjustment of efavirenz is necessary.