Young gay and bisexual men may have difficulty predicting when they’re likely to have sex, which could make it challenging to use on-demand pre-exposure prophylaxis (PrEP). On-demand PrEP involves taking two doses of tenofovir disoproxil fumarate/emtricitabine (Truvada or generic equivalents) two to 24 hours before anticipated sex, one dose 24 hours later and a final dose 24 hours after that. On-demand PrEP can be as effective as daily dosing, but people need to be able to predict when they will have sex so they can take the first dose in time. A study of 120 young (ages 16 to 24) cisgender men who have sex with men found that out of 854 sexual encounters, nearly a third were unpredicted or spontaneous, meaning not anticipated at least two hours in advance. What’s more, 69% of participants had at least one such encounter, meaning they would not be adequately protected by on-demand PrEP. These findings suggest that daily oral PrEP or long-acting injectables might be better options for young men who have spontaneous sex.