On-demand dosing: for anal sex or straight men having vaginal sex

Instead of a daily pill, some people can just use PrEP when needed. This is called on-demand dosing, 2:1:1 dosing or event based dosing.

2:1:1 dosing can be used for anal sex (if you are the active or passive partner). It cannot be used by women, trans and nonbinary people for vaginal or front hole sex.

In people who can use this option, 2:1:1 dosing is just as effective as daily PrEP. It involves taking two pills before sex and two pills after.

2:1:1 dosing is less expensive because you need fewer pills, but can’t be used if you have hepatitis B.

How to take on-demand dosing

  1. Take a double dose of PrEP (two pills) before you have sex. Ideally this should be between 2 and 24 hours before sex.
  2. Take a single pill 24 hours after the double dose.
  3. Take another single pill the following day, 24 hours later.

Aim to take the single pills 24 hours and 48 hours after the first double-dose.

The pre dose is important for the highest protection. If you miss or are late with the pre dose, take the double dose as soon as possible.

  • Never take more than two doses at the same time. Extra doses will not give you any more protection.
  • Only take a double-dose on the first day – not on any other days.
  • If you want to use on-demand PrEP again later in the week, still start with a double-dose, if you went more than a day without taking PrEP. Otherwise, just continue using single doses.

Real life examples of on-demand dosing

Last updated: 1 November 2022.