Q and A


Will taking PrEP late work with on demand dosing?

Hi, I take PrEP with on demand dosing and after having sex after my first double dose i remembered my first dose post sex on time. However, my second dose was 5h late and 3rd dose was 5h late too. Will this affect my level of protection and should i continue taking it 5h late every day?



Thanks for your question. The easy answer is that you will probably be fine.

Both the drugs in PrEP stay in your body to allow for a few hours being late with dosing.

It is better to try to take them at the right time.

The after sex doses with on demand dosing only need to be taken 24 hours and 48 hours after the first double dose. Taking an extra dose for a third day will not do any harm though.

If you are still having sex every day though, then continue taking PrEP until 24 and 48 hours after the last time you had sex.

More information on real life examples for on demand dosing.


  1. Josh Peasegood

    Hi Asher, yes your PrEP will still work properly. Being a few hours late on the third day is not going to cause a problem. As you had doses before and after this, there is no risk of PrEP failing for this reason.

  2. Asher

    So I did on demand prep. Did not have sex the 1st night as expected but continued. This is the timeline:
    Thursday: between 4 and 6pm took 2 pills
    Friday: took pill at 4.
    Had sex at 2 am
    Saturday:was late but took pill at 630pm
    Sunday: took pill at 4pm.

    My question is did it work properly

  3. Josh Peasegood

    Hi John, while being late with the last dose is not ideal, this is still very likely to work. This is because the first 3 pills in your system will still be active.

    The higher initial dose allows for some leeway in being late with the following 2 doses. There is no reason to worry.

  4. John

    Hi, I take prep on demand.

    Sat 2 pills (13:00) had sex at 2am
    Sun 1 pill (13:00)
    Mon 1pill (18:00)

    Wondering if taking the last pill 5h late is something to worry?

    Thanks for your help

  5. Josh Peasegood

    Hi Mr Jones, while your dosing was a few hours outside of recommended dosing. There would still be enough PrEP in your body to be effective.

  6. Mr Jones

    Hi, im taking PrEP on-demand.
    Monday 3am i took the first 2 pill.
    Tuesday 4am (1 pill).
    Wednesday, i missed my schedule, had sex 6am and took 11am (1 pill).
    Thursday 4am (1pill)


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