Does it work for vaginal sex? For vaginal sex, you need to take PrEP every day. This is because PrEP does not get into the vaginal tissues as well as it gets into rectal tissues. Daily dosing keeps the drug levels higher for better protection, and you will need to take PrEP for a week before reaching drug levels that give the highest protection. On-demand or event-based dosing is NOT suitable if you are having vaginal sex.