Disclosed is the use of phorbol esters of Formula I, including 12-O-tetradecanoylphorbol-13-acetate (TPA / phorbol-12-myristate-13-acetate / PMA), for the treatment of HIV/AIDS or neoplasms. In a preferred embodiment the phorbol esters of Formula I are for reducing or controlling resistance to anti-retroviral therapy in an HIV infected mammalian subject with said resistance.