 In combination studies there were synergistic antiviral effects observed between emtricitabine and efavirenz, efavirenz and tenofovir, and emtricitabine and tenofovir.Efavirenz/emtricitabine/tenofovir was approved in the United States as a once-daily tablet to treat HIV in 2006.The main advantage of the new drug was that it could be taken once daily and reduce the overall stress in an antiretroviral regimen.