PEP and PrEP sound almost identical, and people mix them up all the time. Both use HIV medicines to stop infection. The difference is timing: one is taken after something has already happened, the other before anything happens. Getting that right matters, because choosing the wrong one at the wrong moment can leave you unprotected.
Here is a straightforward way to tell which one fits your situation, with examples of the people we see on Koh Samui.
Key points
- PEP is an emergency course after a possible exposure. Start within 72 hours, take it for 28 days.
- PrEP is ongoing prevention for people who may be exposed in future. Taken daily, or around sex for some people.
- If the exposure happened in the last 72 hours, you need PEP first, even if you want PrEP long term.
- Many people move from PEP straight onto PrEP once the course ends.
The one-question test
Ask yourself: has something already happened in the last three days?
If yes, you are in PEP territory. Sex without a condom, a condom that split, a shared needle or an assault within the last 72 hours means you should talk to a doctor about PEP now. PrEP started today would not protect you against an exposure that has already happened.
If nothing has happened, but you expect it might, you are in PrEP territory. That could be a new relationship, a long stay with an active social life, or simply knowing that condoms are not used every time.
Side by side
| PEP | PrEP | |
|---|---|---|
| When you take it | After a possible exposure | Before any exposure |
| How fast to start | Within 72 hours, ideally 24 | No rush, plan it |
| How long | 28 days | As long as you need protection |
| Who it is for | Anyone after a specific risk | Anyone with ongoing risk |
| Tests before | HIV test, sometimes kidney and hepatitis B | HIV test and kidney check, hepatitis B for on-demand |
| Follow-up | HIV tests at about 6 weeks and 3 months | HIV and STI tests about every 3 months |
Four real-world examples
The weekend that went further than planned
Two nights ago, in Chaweng, sex without a condom with someone you met that evening. That is a PEP situation. You are inside 72 hours, so the priority is a doctor today, not tomorrow.
The three-month stay
You have rented a place in Bophut for the season, you are dating, and you know condoms will not always be part of it. Nothing has happened yet. That is a PrEP situation. Start before the risk, not after.
The couple with different HIV statuses
Your partner is living with HIV and has just started treatment. Once their viral load has been undetectable for at least six months, HIV cannot be passed on through sex. Until then, PrEP can give you protection in the meantime. If a condom broke recently, PEP comes first.
The repeat PEP user
You have taken PEP twice this year. That is a strong sign PrEP would make life easier. Rather than a 28-day emergency each time, you would already be protected.
Not sure which one fits?Tell a doctor what has happened and what you expect. They will tell you whether you need PEP now, PrEP for the future, or both.
Can you go from PEP to PrEP?
Yes, and it is one of the most common paths. If the situation that led to PEP is likely to happen again, the doctor can plan for you to start PrEP on the day your PEP course ends. You will have an HIV test first to confirm you are still negative. Starting PrEP straight after PEP means there is no unprotected gap between the two.
Why you cannot just start PrEP after an exposure
It is tempting to think a PrEP tablet the morning after is the same as PEP. It is not. PrEP uses fewer medicines and takes time to build up protective levels in the body. Daily PrEP needs around 7 days to protect for anal sex and around 21 days for vaginal sex. PEP is designed specifically to act after the virus is already in the body, so after an exposure it is the right tool.
Daily or on-demand PrEP
If PrEP is your answer, there are two ways to take it:
- Daily PrEP works for everyone: one tablet at about the same time every day.
- On-demand PrEP, also called 2-1-1, means two tablets 2 to 24 hours before sex, then one tablet 24 hours later and another 24 hours after that. It is only proven for anal sex in cisgender men and is not suitable for people with hepatitis B.
The doctor will help you choose. You can read more on our PrEP page.
What neither of them does
PEP and PrEP only prevent HIV. They do not protect against gonorrhoea, chlamydia, syphilis or hepatitis C, and they do not prevent pregnancy. Regular STI testing is part of the plan with either one, and condoms still add protection against other infections.
Exposed in the last 72 hours?Do not wait to decide about PrEP. Start with PEP today and talk about the long term once you are protected.
Frequently asked questions
Which is more effective, PEP or PrEP?
Both are highly effective when used correctly. PrEP taken as prescribed reduces the risk of getting HIV from sex by about 99%. PEP is very effective when started early and taken for all 28 days, but timing makes a big difference.
Do I need a prescription for either?
Yes. Both need a doctor to check your HIV status and health first. On Koh Samui you can walk in to the clinic any time without a booking.
Can I stop PrEP while travelling?
You can, but how you stop depends on the type of sex you have and how you take it. Ask the doctor before stopping so you stay protected after your last exposure.
Is PEP harder on the body than PrEP?
PEP uses more medicines, so mild side effects like nausea or tiredness are a little more common. Most people manage them without problems, and they usually fade within the first week.