What about on-demand or event-based dosing? Taking PrEP before and after anal sex is still very effective. But the “before-sex” dose is especially important. This is to make sure there are good drug levels before you have sex. On-demand dosing involves: Taking a double dose of PrEP (two pills) before sex. Ideally this is the day before sex (i.e. about 24 hours). PrEP can still be effective if taken between 2 and 24 hours before sex. Taking a single pill 24 hours later. Taking another single pill a further 24 hours later. You should have 48 hours between sex and your last PrEP pill – so keep taking PrEP every 24 hours until this is the case.