Key takeaways
- How effective is PEP? Highly effective when started early and taken for all 28 days, but not 100%.
- Guidelines advise starting as soon as possible, preferably within 24 hours, and not after 72 hours.
- No study gives a reliable percentage for each hour of delay.
- PEP usually fails because of a late start, missed doses, further exposures or HIV already present.
- Start quickly, finish the course, then complete your follow-up tests.
How effective is PEP at preventing HIV?
When it is started quickly and taken correctly for 28 days, PEP is very effective at preventing HIV, and true PEP failures are rare. It is not a guarantee, though, and how well it works depends mostly on how soon you start and whether you take every dose.
Below is what the evidence shows, what it cannot show, and what you can control.
Why no one can give you an exact PEP effectiveness percentage
You may see figures such as “80%” quoted online. None of them is a precise PEP effectiveness rate for your situation, and here is why.
There are no randomised trials of PEP after sex
The UK national guideline explains that trials giving some people PEP and others a dummy pill have not been done, because withholding a potentially effective treatment would be unethical.
What the evidence is based on instead
- Animal studies: PEP works best within 24 hours, may still work up to 72 hours, and needs about 28 days.
- Health workers exposed at work in the 1980s and 1990s: a single older medicine was linked to about an 80% reduction in infections.
- People who took PEP after sex: infections were uncommon and usually linked to later exposures.
The “80%” comes from that older study. Modern three-drug PEP is more potent and easier to take, so it is expected to do better, but no exact figure has been measured.
PEP effectiveness within 24, 48 and 72 hours
Every guideline agrees that earlier is better. The UK guideline for the use of HIV post-exposure prophylaxis recommends starting as soon as possible, preferably within 24 hours, and not starting beyond 72 hours.
The evidence cannot give an exact PEP effectiveness percentage at 36 or 48 hours. Figures you may see for different delays come from computer models built on assumptions, not measurements in people.
What the evidence shows by time since exposure
| Time since exposure | What the evidence shows | What to do |
|---|---|---|
| Within a few hours | The best chance | Go to a clinic now |
| Within 24 hours | The preferred window in national guidelines | Start today |
| 24 to 48 hours | Still recommended and likely to work | Start as soon as possible |
| 48 to 72 hours | Still recommended; evidence is weakest | Go immediately |
| After 72 hours | Not recommended; benefit is unlikely | Get tested and advised instead |
If you are past the limit, our guide to options when it has been more than 72 hours since exposure explains what happens next.
Is PEP still effective after 48 hours?
Yes, PEP is still recommended between 48 and 72 hours. PEP effectiveness after 48 hours is probably lower than within 24 hours, but nobody can put a reliable number on the difference.
Do not let a number you read online stop you from going. If you are unsure whether your situation needs PEP at all, the 72-hour PEP window calculator and self-check can help you decide quickly.
What makes PEP more or less likely to work
The UK guideline lists the main reasons people have acquired HIV despite starting PEP. Most are things you can influence.
Factors that help PEP work vs factors linked to failure
| Factor | Helps PEP work | Linked to PEP failure |
|---|---|---|
| Timing | First dose within hours | Starting late in the 72-hour window |
| Taking doses | Every dose, at a similar time each day | Several missed or late doses |
| Length of course | All 28 days completed | Stopping early |
| Further exposures | Condoms used during and after PEP | New condomless exposures during or after the course |
| Status at the start | Negative baseline HIV test | HIV infection already present when PEP began |
If you forget a tablet, our guide on what to do if you miss a PEP dose explains how to get back on track without doubling up.
How to give PEP the best chance of working
- Start quickly. Go as soon as you can, even late at night.
- Take every dose. Link it to a daily habit, such as breakfast.
- Finish all 28 days. Animal studies show shorter courses protect less well.
- Manage side effects rather than stopping. Most are mild; see our tips on coping with PEP side effects during the course.
- Avoid new exposures. Use condoms until your follow-up tests are complete.
For how PEP works in the body and which medicines are used, read our full guide on what PEP is and how the 28-day course works.
How you will know whether PEP worked
Feeling well, or having side effects, does not tell you whether PEP worked. The answer comes from follow-up HIV testing, usually around 4 to 6 weeks and again at about 3 months after the exposure.
Our article on HIV testing after finishing PEP explains the timing and what each result means.
Does PEP effectiveness depend on the type of exposure?
Studies have not measured PEP effectiveness separately for each type of exposure. What differs is the starting risk, which is why doctors recommend PEP for some situations and not others.
For your own starting risk, see our guide to your HIV risk when a condom breaks explains the chances by type of sex. Australia’s national PEP guidelines from ASHM also stress that each exposure is assessed case by case.
Getting PEP quickly in Bangkok
At HIV PEP Bangkok, near BTS Asok and MRT Sukhumvit, you can walk in for PEP any day from 09:00 to 21:00 and see an English- or Thai-speaking doctor in a private room.
After a consultation and baseline tests, most people take their first dose at the clinic within about an hour and leave with the full 28-day supply. Outside our hours, go to a hospital emergency department; in a medical emergency, call 1669.
If you are still deciding what to do first, read the step-by-step plan for the first 72 hours after a possible HIV exposure.
Frequently asked questions about PEP effectiveness
Is there a reliable percentage for how effective PEP is at 48 hours?
No. Figures for 36 or 48 hours usually come from computer models, not human trials. The evidence says earlier is better, and PEP is still recommended up to 72 hours.
Does taking PEP for fewer than 28 days still protect me?
It may give less protection. Animal studies found shorter courses protected fewer animals, which is why every guideline recommends the full 28 days.
Can PEP fail even if I started on time and took every dose?
It is possible but rare. Most infections after PEP were linked to later exposures or to HIV already present when PEP began.
Are newer PEP medicines more effective than older ones?
Modern three-drug regimens are more potent and easier to take than the single medicine in older studies, and easier courses are completed more often.
If you think you may need PEP, the most important thing is to start quickly. You can walk in, or contact our Bangkok clinic by LINE, WhatsApp or phone and we will help you get your first dose as soon as possible.
