PEP vs PrEP: what’s the difference and which do you need?

PEP vs PrEP: what is the difference?

PEP and PrEP use similar HIV medicines, but they are designed for very different moments. The simplest way to remember it is by the first letters: post means after, and pre means before.

PEP (post-exposure prophylaxis) is a 28-day course started as soon as possible after a single possible exposure, such as a condom breaking. It is a safety net for an emergency. If you want the full picture of how it works, read our guide on what PEP is and how it prevents HIV.

PrEP (pre-exposure prophylaxis) is taken by HIV-negative people before any exposure happens, so there is already enough medicine in the body to stop HIV if it enters. It is a planned, ongoing form of protection, much like the contraceptive pill is for pregnancy.

PEP and PrEP compared side by side

	PEP	PrEP
When you take it	After a possible exposure	Before possible exposures
Time limit	Start within 72 hours, ideally within 24	Start at a planned time, after an HIV test
How long	28 days	As long as you have ongoing risk
Typical medicines	Three HIV medicines, often tenofovir, lamivudine or emtricitabine, and dolutegravir	Usually two medicines in one tablet, or a long-acting injection where available
Best for	A one-off, unexpected event	Repeated or expected risk
Before starting	Rapid HIV test and baseline checks	HIV test, kidney check and STI screen
Follow-up	HIV tests at about 4–6 weeks and 3 months	HIV, kidney and STI checks at regular visits
Protects against other STIs?	No	No

Neither PEP nor PrEP prevents pregnancy, syphilis, gonorrhoea, chlamydia or other STIs. Condoms remain the best way to reduce those risks.

Which one do I need right now?

This is the question that matters most, and the answer usually depends on timing.

• Something happened in the last 72 hours. PrEP started now will not protect you from an exposure that has already happened. You need to be assessed for PEP. Follow our step-by-step plan for what to do after a possible HIV exposure.
• Nothing has happened, but you expect to be at risk. PrEP is the right conversation to have.
• You are not sure whether an event counts as an exposure. Our PEP self-check and risk guide explains which situations usually need PEP and which do not.
• The exposure was more than 72 hours ago. PEP is no longer recommended. Get tested at the right time and ask about PrEP for the future.

Who is PrEP for?

PrEP suits HIV-negative people who have a realistic chance of coming into contact with HIV. You might consider it if:

• you sometimes have sex without condoms with partners whose HIV status you don’t know
• your partner has HIV and is not yet on treatment or does not have an undetectable viral load
• you have had an STI recently, which can be a sign of ongoing risk
• you have used PEP, especially more than once
• you share injecting equipment
• you are travelling or moving somewhere and expect new partners

If your partner has HIV and has had a sustained undetectable viral load on treatment, they do not pass HIV on through sex (undetectable equals untransmittable, or U=U). Some couples still choose PrEP for peace of mind while that is being confirmed.

What types of PrEP are there?

Daily oral PrEP

The most common option is one tablet taken every day, usually a combination of tenofovir and emtricitabine. It is suitable for people of all genders and for all types of sex. The key is consistency: PrEP works very well when it is taken as prescribed, and much less well when doses are regularly missed.

Event-driven “2-1-1” PrEP

Event-driven PrEP, also called on-demand PrEP, is an option recommended only for cisgender men who have sex with men. It involves a double dose taken before sex, then one tablet 24 hours after the double dose and another tablet 24 hours after that. It suits people who have sex less often and can plan ahead.

It is not suitable for vaginal or front-hole sex, for trans people using hormones, or for people with hepatitis B, so a doctor must check whether it is right for you.

Long-acting injectable PrEP

In some countries, PrEP is also available as an injection given every few months instead of a daily tablet. This can help people who find daily pills hard to remember. Availability and cost vary from country to country and change over time, so ask a clinic what is currently offered where you live.

How long does PrEP take to start working?

PrEP does not protect you from the first tablet. The medicine needs time to build up to protective levels in the body, and the time needed depends on the type of sex and the way you take PrEP. Your doctor will tell you exactly when you can rely on it and how to take the first doses.

Until then, use condoms or other protection. If an exposure happens before PrEP has had time to work, contact a clinic about PEP straight away.

Can I switch from PEP straight to PrEP?

Yes, and for many people this is the best plan. If you needed PEP and expect similar situations in the future, you do not need to stop and wait. Many doctors recommend moving from the last day of PEP onto PrEP with no break, so there is no gap in protection.

Before switching, the clinic will usually:

1. confirm that you are still HIV-negative with a test
2. check your kidney function, as PrEP is taken over a longer period
3. review any STI results and your hepatitis B status
4. talk through which type of PrEP suits your life

You will still need your usual follow-up HIV tests after PEP. Our guide on when to test for HIV after an exposure explains the timing.

What do PrEP check-ups involve?

PrEP is not a one-off prescription. Regular visits keep it safe and effective: HIV.gov’s PrEP guide notes that people on daily PrEP pills see a provider about every 3 months for HIV tests and refills. A typical check-up includes:

• An HIV test, because PrEP should not be taken by someone who has HIV
• A kidney check, as some PrEP medicines can affect kidney function in a small number of people
• An STI screen, including syphilis, gonorrhoea and chlamydia
• A chat about side effects and how easy you find it to take PrEP
• A new supply of tablets

Most people have mild side effects or none at all. If you have been on PEP and coped well, PrEP usually feels easier, as it involves fewer medicines.

Getting PEP or PrEP in Bangkok

At our clinic near BTS Asok and MRT Sukhumvit, you can walk in for PEP any day from 09:00 to 21:00. For PrEP, we offer screening, a kidney check and a 30-day supply, with English- and Thai-speaking doctors and private consultation rooms. If you are visiting Thailand, our sexual health guide for travellers in Bangkok covers bringing and topping up PrEP on a trip.

Frequently asked questions about PEP vs PrEP

Can I take PrEP instead of PEP after an exposure?

No. PrEP only protects against exposures that happen after it has built up in your body. If the exposure has already happened, you need to be assessed for PEP within 72 hours.

Is it safe to stop PrEP?

Yes, many people stop when their risk changes. How you stop depends on the type of PrEP and the type of sex you have, so ask your doctor for a plan before stopping.

Does PrEP work for women?

Yes. Daily PrEP works for people of all genders when it is taken consistently. Event-driven 2-1-1 dosing is not recommended for vaginal or front-hole sex.

Can I take PrEP if I have hepatitis B?

Sometimes, but it needs careful planning. Some PrEP medicines also act on hepatitis B, so stopping suddenly can cause it to flare, and event-driven dosing is not suitable. A doctor will check your hepatitis B status first.

Do PEP and PrEP use the same pills?

They overlap. PrEP tablets usually contain two of the medicines also used in PEP, but PEP normally adds a third medicine because it has to stop an exposure that has already happened.

Not sure which option suits you? Compare our PEP and PrEP services in Bangkok, or message us to book a confidential consultation and a doctor will help you decide.