The PEP 28-day course is a month of daily HIV medicines taken after a possible exposure to help stop HIV from taking hold. It must start within 72 hours, ideally within hours, and it works best when you take every dose for the full 28 days.
This guide walks you through each week: what you may feel, what to do if you miss a dose, and what happens after your last pill. Your doctor will give advice for your own medicines.
Why PEP lasts 28 days
HIV needs time to set up in the body after an exposure. PEP tries to block it during that time, and studies have settled on four weeks as the standard course.
The US CDC's clinical guidance on PEP recommends a 28-day course for everyone offered PEP. Taking it for longer doesn't make it work better.
Is PEP 28 days or 30?
The course is 28 days. Some packs contain 30 tablets, so you may have a couple left over.
Don't keep taking extra pills unless your doctor tells you to. If you're unsure when to stop, message them.
What to expect week by week
Everyone's experience is different. Many people have mild side effects or none at all, and most side effects fade as the body gets used to the medicine.
The 28-day PEP course, week by week
| Week | What usually happens | What to focus on |
|---|---|---|
| Week 1 (days 1–7) | You start PEP after a baseline test; mild nausea, tiredness or headaches may appear after a few days | Build a daily routine; message your doctor about side effects |
| Week 2 (days 8–14) | Side effects often ease; some people feel back to normal | Keep taking every dose; don't stop without advice |
| Week 3 (days 15–21) | Motivation can drop because you feel fine | Keep your alarms on; check you have enough pills |
| Week 4 (days 22–28) | The final stretch; your last dose is on day 28 | Book your follow-up HIV tests |
| After day 28 | Medicine stops; side effects usually go within days | HIV tests at about 4–6 weeks and ~3 months |
For more on how you might feel, see our guide to PEP side effects and when to call a doctor.
Missed a PEP dose? What to do
Missing a dose happens, especially when travelling. It's not a reason to panic or to give up.
- Don't double up. Never take extra pills to catch up unless your doctor says so.
- Message your doctor. Tell them which dose you missed and when you realised.
- Carry on with the course. Stopping altogether is the bigger risk.
The UK charity i-base explains in its PEP questions and answers that being a little early or late is usually fine. Your doctor can tell you what applies to your medicines.
If you vomit after a dose
Tell your doctor, especially if it happens more than once. They may suggest a way to ease nausea or adjust your plan.
Finishing the course vs stopping early
Some people want to stop because they feel sick, feel fine or think the risk was small. Please talk to a doctor first.
Finishing vs stopping early: pros and cons
| Choice | Pros | Cons |
|---|---|---|
| Finish all 28 days | Gives PEP its best chance to work; matches guidelines | Four weeks of daily routine; possible mild side effects |
| Stop early on your own | Side effects end sooner | PEP may not work as well; no medical check |
| Stop early on medical advice | Right when a doctor finds PEP isn't needed | Only after a proper review |
A doctor may stop PEP early if new information shows the risk was very low. That decision should be theirs, not a guess.
Feeling fine is not a reason to stop
Feeling well in week three is a good sign that your body has settled. It doesn't tell you anything about whether HIV was blocked.
The same goes for feeling unwell. Tiredness or a headache is far more likely to be a side effect or a normal bug than a sign of HIV. Keep going, and let your doctor judge any symptoms that worry you.
Your daily PEP routine checklist
A simple routine makes 28 days much easier. Use this checklist, especially if you're moving between islands.
- Take your dose at the same time each day
- Set two alarms on your phone
- Keep pills in your day bag, not only your room
- Carry a bottle of water
- Tick off each day on a calendar or app
- Keep a photo of your prescription on your phone
- Tell your doctor about any other medicines or supplements
- Go easy on alcohol, which can make you forget doses
Travelling during the course
Make sure you leave the clinic with the full 28-day supply. Keep it in your hand luggage when flying.
Our follow-up support after PEP, including a summary for your home doctor helps you finish the course at home.
PEP after 28 days: what comes next
When you take your last dose, the course is done. You don't need to taper off.
Next come your follow-up tests. Most doctors suggest an HIV test at about 4–6 weeks and again at about 3 months. Read HIV testing after PEP: when to test and how accurate it is for the details.
New risk while on PEP?
Tell your doctor straight away, even if it's near the end. They'll tell you whether your plan needs to change.
If you've needed PEP more than once, it's worth reading about how PEP compares with PrEP and condoms and asking whether PrEP suits you.
Support during your PEP course on Koh Samui
You don't have to get through the 28 days alone. Our HIV PEP service on Koh Samui includes advice on taking your medicine and handling side effects.
After the course, our follow-up support helps you book tests and understand results. Wondering about the cost of a full course? See what a 28-day PEP course costs on Koh Samui, or message us for our current price.
Frequently asked questions
Can I drink alcohol while on PEP?
A small amount isn't known to stop PEP working, but alcohol can worsen nausea and make you forget doses. Keep it light and ask your doctor.
What happens if I miss one PEP dose?
Don't double up. Message your doctor, then carry on with the course as advised.
Can I stop PEP after two weeks?
Only if a doctor advises it. Stopping early on your own may make PEP less effective.
Do I need to continue PEP after 28 days?
Usually not. The course is 28 days, and your doctor will tell you if anything is different for you.
Can I take other medicines or supplements with PEP?
Some can interact, including certain antacids, vitamins and minerals. Tell your doctor about everything you take, including anything bought at a pharmacy, before you start.
When should I test after finishing PEP?
Most doctors suggest about 4–6 weeks and about 3 months after the exposure. Your doctor will confirm the dates.
Starting or struggling with your PEP course?
If you haven't started yet, remember that PEP must be started within 72 hours — the sooner the better. If you're part-way through and something feels off, we're here to help.
Message a doctor privately on WhatsApp about your 28-day PEP course.