the initial form of tenofovir used in these studies had limited potential for widespread use because it poorly penetrated cells and was not absorbed when given by mouth. gilead developed a pro-drug version of tenofovir, tenofovir disoproxil. this version of tenofovir is often referred to simply as "tenofovir". in this version of the drug, the two negative charges of the tenofovir phosphonic acid group are masked, thus enhancing oral absorption.