Using intraerythrocytic tenofovir-diphosphate data from the F-TDF arms of HPTN 083 (men) and 084 (women), ∼99% efficacy was achieved for the highest adherers in both studies, but at a lower adherence threshold in 083 (≥2 doses/week) as compared with 084 (daily), suggesting higher adherence is necessary for women versus men. Keywords: PrEP; adherence; pharmacology; sex differences; tenofovir.