In 2010 we learned that taking a daily dose of an HIV medication can protect people from getting HIV through condomless sex. This is known as PrEP (pre-exposure prophylaxis). A study called iPrEx showed that PrEP works – as long as you take it as prescribed. A follow-up study found there were no HIV infections among participants who took at least four pills per week.

Some folks asked: what about only taking pills when you’re having sex? Another study called IPERGAY answered that question. Two hundred guys in France and Montréal took pills before and after sex. (We’ll show you the timing schedule below!) Known as ‘event-based’ dosing, this approach proved to be just as effective as daily dosing. Just like daily dosing, event-based dosing (EBD for short) only works when you take it.

As it happens, participants in the IPERGAY study were having a lot of sex. At least half were taking four pills or more per week – about the same amount that proved 100% effective with daily dosing. So the IPERGAY team also looked at data from folks having less ‘busy’ months, when they were using event-based PrEP less often. Result: EBP was still just as effective as daily dosing.
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You need to leave at least two hours’ lead-in before taking the startup dose and having sex – enough time for the pills to start working.

For example, if your sex life involves the occasional binge or blowout, like a sauna visit after work or a party weekend, or if you generally arrange hookups a couple of hours before meeting up, EBP may be an option for you to consider.

Other reasons for using EBP might include wanting to get more bang for your buck – reducing the overall cost of PrEP by only taking pills when you’re actually having sex. A small number of people experience continuing side effects during daily dosing, and using EBP might help you reduce the amount of time you experience those side effects.

If you are thinking of doing EBD, you also need to consider:

• Do you normally have at least two hours lead-in before sex?
• Are you generally pretty good at taking pills on time?
• Can you handle the additional requirements of EBD?

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When you start PrEP, you get tested for HIV and STI (sexually transmitted infections) and a kidney function test. It’s important to check that you don’t have HIV before you start PrEP, because the two drugs used for PrEP are not enough on their own to suppress HIV infection, and taking PrEP when you have HIV can limit some of your options for future treatment.

You also need follow-up STI checkups every three months, because PrEP only prevents HIV. Regular STI testing and treatment also help reduce the prevalence of STIs in the community. You should get vaccinated for hep A and HPV (if you are eligible), and it’s important to check that you don’t have hep B before using PrEP.

If you are thinking about, or have already started doing EBP, we recommend letting your doctor know so that they can support you. We have included key references and guidelines below, in case your doctor is not convinced about EBP. Show less