iPrEx (from Spanish: Iniciativa Profilaxis Pre-Exposición, "pre-exposure prophylaxis initiative") was a phase III clinical trial to determine whether the antiretroviral medication emtricitabine/tenofovir (as tenofovir disoproxil fumarate) could safely and effectively prevent HIV acquisition through sex in men who have sex with men and transgender women. iPrEx was the first human study of an HIV prevention strategy known as pre-exposure prophylaxis, or PrEP.