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

For people who do not want to take a daily pill there is an option to just use PrEP when you need it. This is called on-demand dosing (or event-based dosing).

Just taking PrEP before and after a risk is very effective. This option is important if you don’t often have anal sex without condoms. Also, if you usually know when you will have sex.

If you are buying PrEP, on-demand dosing will be less expensive because you need fewer pills. However, on-demand dosing can’t be used if you have hepatitis B.

On-demand dosing involves:

  1. Taking a double dose of PrEP (two pills) before you have sex. Ideally this should be between 2 and 24 hours before sex as this dosing was used in the IPERGAY study.
  2. Taking a single pill 24 hours after the double dose.
  3. Taking another single pill the following day, 24 hours later.

You should aim to take a single pill 24 hours and 48 hours after the first double-dose.

Although the pre dose is important for the highest protection, if you miss or are late with the pre dose, taking the double dose as soon as possible will still give some protection.

The next two pages show examples of on-demand dosing.

Last updated: 1 November 2019.