PEP: your questions answered…
This page answers your questions about PEP.
The risk of catching HIV from one exposure is usually less than 1 in 100 (1%). Often it might be less than 1 in 1,000 or 1 in 10,000.
There is ZERO risk if your partner is taking effective HIV treatment.
- What is PEP?
- Does PEP work?
- How does PEP work?
- How well does PEP work?
- How soon after sex do you need to take PEP?
- How many drugs are in PEP?
- Which drugs are used for PEP?
- Can efavirenz or Atripla be used in PEP?
- What if the HIV is resistant to the PEP drugs?
- How long do you need to take PEP for?
- What if I am late or if I miss a dose? Can I change the dose time?
- Can I stop earlier than 28 days?
- What if I get symptoms when taking PEP?
- What if I get symptoms after stopping PEP?
- What if I am in a country where it is difficult to test?
- When is PEP used?
- When can I test to see if I am HIV positive?
- Can I have sex when on PEP?
- Can I masturbate when on PEP?
- If I do have sex while taking PEP, do I need to extend PEP for another 28 days?
- If I do have sex on PEP, does my partner need PEP too?
- What about DoxyPEP for other STIs?
- What is the main difference between PEP and PrEP?
- How can I get PEP?
- What else is involved in taking PEP?
- Why is there so little evidence about well PEP works?
- What is the difference between PrEP and PEP?
- Does PEP interact with other medications?
- Does food, drink, alcohol, weed or cigarettes affect PEP?
- Do supplements/multivitamins affect PEP?
- Should I continue PEP if I find out my partner is negative?
- Can I change the bottle used for PrEP?
- What if I find out my risk partner is HIV negative?
- References and links for further info
1. What is PEP?
PEP is a way to use HIV meds AFTER sex to try to prevent HIV infection.
This usually involves taking a combination of three daily oral meds for 28 days. Sometimes two or three of these meds will be in a single pill.
Any 3-drug ART combination can be used as PEP so long as it does not contain abacavir or an NNRTI. This can include drugs that are not suggested by local guidelines e.g, Biktarvy.
HIV PEP is only effective against HIV.
PEP is NOT a contraceptive and it does NOT prevent other infections.
PEP is for emergency use. If you need PEP then PrEP might be a good option for the future.
2. Does PEP work?
Yes, PEP can stop and reduce the risk of HIV.
Starting as soon as possible gives PEP the best chance to work. PEP is most effective if taken within 24 hours.
PEP can only work though if you take it, especially for the first week. If you miss doses this could reduce the chance it will work,
Current guidelines recommend taking the full course as prescribed.
3. How does PEP work?
It takes a little time for HIV infection to take hold. This might take a few hours or a few days.
This leaves a short window for PEP to work in two ways.
- Very early PEP might stop HIV from infecting any cells.
- It might also stop HIV from spreading beyond the first infected cells. These cells would then die without infecting any others.
Having good levels of HIV drugs at the right time can prevent infection from becoming established.
4. How well does PEP work?
PEP is very effective.
Very few people test positive after using PEP. This just involves remembering to take the meds each day at roughly the same time.
Defining how well PEP works is more difficult, but also will never be clearly known.
This is because it is a very difficult thing to study. Firstly, HIV is not a very infectious virus. Even after higher-risk sex, this chance of becoing HIV positive is is much less than 1 in 100 (less than 1%).
It would also be unethical to not give PEP to anyone who needs it. So any study would not have a non-PEP comparison group to compare it to.
The earlier you start PEP, the more likely it will work.
- UK guidelines say PrEP is most effective when started within 24 hours, but that it might have some effect up to 72 hours.
- US guidelines say “every hour counts”. Also, that PEP might reduce the risk of HIV by more than 80% if taken as prescribed.
5. How soon after sex do you need to take PEP?
The sooner you take PEP, the sooner it can work.
- Start as soon as possible.
- This should ideally be within a few hours, or at least on the same day.
- In practice it usually takes 24 hours to start.
- The earlier it is started the earlier it can start to work.
- PEP will not work after 72 hours (3 days).
6. How many drugs are in PEP?
PEP uses three drugs.
- Sometimes all three drugs are in one daily pill.
- Sometimes PEP involves taking these meds as separate pills.
- Only using two drugs will not be as effective as three. However, two drugs is better than none, if started early.
What if I was only given 2-drug PEP?
- PEP with only two drugs needs to be taken within 24 hours of sex..
- It is much less likely to be effective if taken later than this.
- If a third drug becomes available, this should immediately be added.
7. Which drugs are used for PEP?
In the UK, PEP currently uses tenofovir/emtricitabine + raltegravir.
- These meds need to be taken once a day, at roughly the same time every day for 28 days.
- This combination can be taken with or without food.
Different drugs are used in different countries.
High income countries often use Biktarvy. This contains tenofovir/emtricitabine/bictegravir in one small tablet.
Low and middle-income countries are increasingly using TLD (sometimes called LTD). This contains tenofovir/lamivudine/dolutegravir in one tablet.
8. Can efavirenz be used in PEP?
In the UK, combinations that include efavirenz should NOT be used as PrEP.
This is because side effects of efavirenz include mood changes like anxiety and depression. There is also a very low risk of serious liver toxicity. To reduce side effects, efavirenz needs to be taken before bedtime and not within two hours of a high fat meal.
These combinations would still work against HIV, but the side effects might make your quality of life very difficult during this month.
Even so, efavirenz-based PEP is still used in some countries, usually as a generic formulation of Atripla.
9. What if the HIV is resistant to the PEP drugs?
If the HIV is resistant to one of the PEP drugs, PEP is still likely to work. This is because the other drug(s) will still be active.
If the HIV is resistant to all three PEP drugs, then PEP will not work. However, this is very rare, even in countries where drug resistance is more common.
10. How long do you need to take PEP for?
Oral PEP needs to be taken every day for 28 days.
- It is best to take PEP meds together at roughly the same time each day.
- PEP can be taken with or without food.
- Sometimes PEP comes in a box of 30 doses. It does not matter whether or not you take these extra few days.
- Do not take PEP for longer than 30 days. Taking PEP for longer doesn’t make it any more effective. Even if yu have another risk while on PEP, you do not need to take it for another month.
- If you are worried about HIV in the future, please take to a doctor about using PrEP.
11. What if I am late or if I miss a dose? Can I change the dose time?
If you forget a dose, just take it when you remember. But if you miss by a whole day, ONLY take a single dose. Do NOT double up for the missed dose.
- It is also okay to be a couple of hours early or late sometimes. The main thing to to try to take PEP every day.
- Even if you do miss a day this is still likely to be okay. If you missed a dose, then you missed it.Try your best to remember the others.
- Aiming for the same time each day will be an easier routine to remember.
- Most of the benefit from PEP comes from starting early and good dosing for the first few weeks.
- Even if you miss doses it is still good to continue to finish the course of PEP.
- There is no data to estimate any changes in risk from missing specific times or days.
- Generally, adherence is most critical earlier on in the course.
It is easy to change the time you take PEP if you decide a different time of day is better for you.
12. Can I stop earlier than 28 days?
PEP is given for 28 days.
- If you stop earlier, it might not work as well.
- A shorter course of PEP might work, but needs to be studied.
- The first few weeks of PEP are the most important.
- Please talk to the doctor who prescribed PrEP if you decide you want to stop.
- i-Base are not doctors. We cannot tell you to either start or stop PEP.
13. What if I get symptoms or side effects when taking PEP?
Symptoms while taking PEP are NOT signs of HIV.
- HIV symptoms come from having a high viral load and PEP would stop a high viral load.
- Symptoms might be the result of stress. For example, if you are not sleeping well because you are worrying about HIV.
- Symptoms might be a side effect of PEP. These are generally mild and tolerable. If the symptoms are difficult, please contact your doctor.
- Any rash that is more than very mild is always important to show the doctor iin case this is a rare but serious allergic reaction.
In the UK, more than 99% of HIV tests come back negative. It is very rare for a test to be positive after using PEP.
PEP is very unlikely to cause serious side effects.
Many people do not get any side effects or they are mild and quickly resolve.
Efavirenz does cause side effects though (see below). This is why efavirenz should not generally be used for PEP.
14. What if I get symptoms after stopping PEP?
- Getting symptoms after stopping PEP does NOT mean you are HIV positive.
- Usually, feeling tired, or having a cough, or a headache, will just be feeling tired and having a cough or a headache.
- Stress can also cause symptoms, especially if you are not sleeping well. One or two symptoms are unlikely to be HIV.
- Seroconversion symptoms generally start about 10 to 14 days after exposure. If you have been on PEP, they would occur 10 to 14 days after the PEP ends.
- Seroconversion is often like a very heavy flu that lasts for about a week and then resolves.
- Even you have similar symptoms, this does not mean it is HIV. It might, for example, be COVID. Please talk to a doctor if any symptoms are serious.
- Taking another HIV test six weeks after stopping PEP will confirm whether or not this is HIV.
See: What is seroconversion and what are the symptoms?
After 6 weeks, a 4th generation antigen/antibody HIV test (Ag/Ab) confirms whether this is HIV or not.
Testing HIV negative after only 3 or 4 weeks is good news, but needs to be repeated.
Six weeks is used in the UK because more than 95% of people will get a positive result by this time. The UK is also a low incidence country, with relatively few infections each year. Some guidelines in other countries might still recommend a confirming test after 12 weeks (3 months).
A viral load (RNA) test after 2 weeks can help know if symptoms are related to HIV. Seroconversion symptoms are because viral load is very high. So a negative viral load test while you have symptoms, means this is unlikely to be HIV.
Viral load tests are not approved to diagnose HIV. They are also more expensive than HIV tests so are not routinely available on the NHS. Sometimes they might be available in cases of assault or other traumatic circumstances.
15. What if I am in a country where it is difficult to test?
Every country should have some clinics where testing is free or cheap.
If you can’t access these because or your immigration status or job, you could buy a self test online that you can do at home. These are usually 3rd generation tests. Or use a self sample service where you have to post this back to get a result.
Or plan a trip to a country where is is easy to test.
16. When is PEP used?
PEP is only used if you had penetrative sex.
- With someone whose HIV status you don’t know, or
- With someone who is HIV positive and who has detectable viral load.
PEP is NOT needed if your partner is HIV positive and on effective treatment.
PEP is NOT needed if you used a condom or if your only risk was oral sex.
17. When can I test to see if I am HIV positive?
Nearly everyone who uses PEP with be HIV negative. But you need to wait at least 4-8 weeks AFTER finishing PEP before testing.
The time varies for different types of test you use. It also depends on if you want to test twice or only once.
4th generation HIV tests (antigen/antibody) are 95% accurate after 4 weeks and more than 99% accurate after 6 weeks. UK guidelines recommend waiting 6 weeks which will not need to be repeated.
If you only wait 4 weeks, the result needs to be confirmed a month later with a second test.
If you use a 3rd generation test (antibody only), please wait 8 weeks for a final result. Testing earlier than this means you need to confirm the result again after 8 weeks.
Please wait at least four weeks after finishing PEP before taking an HIV test.
If you are negative after four weeks, you need to take a second test later to confirm the result.
The UK recommends waiting six weeks after finishing PEP.
If you are negative after six weeks you do not have to take a second test.
18. Can I have sex when on PEP?
Please take a break from sex while taking PEP.
- This is because of the risk to your partner(s).
- if you were at risk for HIV you were also at risk of other STIs.
- Testing for other STIs involves waiting for a couple of weeks.
- Technically, PEP should protect both you and other partners from HIV though, so long as you are lucky and it is working well.
- This is mainly because PEP is likely to contain the same drugs used in PrEP.
- The highest risk to your partners will be the six weeks after you stop PEP and are waiting for the result of your HIV test.
- If your recent partner is not your regular partner, this will involve not putting your regular partner at risk.
19. Can I masturbate when on PEP?
Yes, you can do this as much as you like.
It will not have any effect on PEP.
20. If I do have sex while taking PEP, do I need to extend PEP for another 28 days?
No. If your PEP includes tenofovir and either lamivudine or emtricitabine, the PEP will also be working as PrEP.
If you are a man, you need to have two days of PEP left to cover the recent sex. If you are a woman, or are trans, you need to have six days of daily PEP left to cover this.
However, please talk to a doctor about using PrEP in the future. PrEP should be offered to anyone who uses PEP.
PEP is used after an exposure. It is used when prevention was not possible. If you are finding that you require regular courses of PEP, consider PrEP.
PrEP is taken daily or event-based and is used BEFORE exposure. It is more effective than PEP and uses fewer drugs so will cause fewer side effects.
Please see these links for information on PrEP.
21. If I do have sex on PEP, does my partner need PEP too?
No.
This is because while you are on PEP you there is not a risk of HIV. This is because PEP reduces any viral load even if you do have HIV.
After your PEP is finished, you need to use condoms with your partner until you get you HIV results. PEP is nearly always effective though.
PEP doesn’t stop other STIs though. If you are on PEP for HIV then you could have other STIs which you could give to your partner. This is another reason why people on PEP are advised to not have sex.
22. What about DoxyPEP for other STIs?
Some researchers are looking at using antibiotics as PEP for STIs.
This is mainly in research studies using an antibiotic called doxycycline.
- These studies use a single 200 mg pill, taken within 72 hours of having sex.
- STI PEP can reduce the risk of some other STIs, but infections can still happen.
- STI PEP is not currently recommended in the UK.
- People are asked to not have sex while on STI PEP.
23. What is the main difference between PEP and PrEP?
PEP is only taken after sex. It also includes three active drugs.
PrEP involves taking HIV meds before and after sex. PrEP only involves two active drugs, in one pill.
A new type of PrEP uses an injection every 2 months. This is not yet widely available because of a much higher price.
24. How can I get PEP?
PEP can only be prescribed by a doctor.
In the UK PEP is available from:
- Any sexual health clinics.
- Accident and Emergency (A&E) departments at a hospital.
- Sometimes from a GP.
Find a clinic in your area: /.
25. What else is involved in taking PEP?
Everyone needs to have an HIV test before taking PEP.
- This is to check you are not already HIV positive.
- This test only tells you your HIV status a month ago. It tells you nothing about your recent risk.
26. Why is there so little evidence about well PEP works?
Most of the best evidence about PEP comes from animal studies.
- These prove that PEP works.
- Human studies about PEP would not be ethical. Once someone needs PEP you can’t withhold it.
- The risk of catching HIV is also generally so low that any study would need to have thousands of participants, to see even a few infections.
- Most observational PEP studies only have a few hundred people. Even if no-one used PEP this is too small to see even 1 or 2 infections.
27. What is the difference between PrEP and PEP?
PrEP is taken BEFORE an exposure and PEP is taken AFTER.
- PrEP can be taken daily or on-demand. PEP is taken daily for 28 days.
- Taking PrEP AFTER is not effective. It can be taken daily until you have access to PEP.
- PrEP meds can be used in PEP combinations. PEP should not be used as PrEP.
28. Does PEP interact with other medications?
As with other drugs, PEP has the potential to interact with other types of medicines.
It can also sometimes interact with drugs you can buy over-the-counter without a prescription and sometimes with supplements.
Please tell your doctor about any other medicines or supplements that you use. You can also talk to any pharmacist.
Interactions will depend on the individual drugs in your PEP. This can also be checked on the Liverpool University HIV drug interaction website.
If in doubt about non-prescribed supplements, it is easy to just stop these, as PEP is only four weeks.
29. Does food, drink, alcohol, weed or cigarettes affect PEP?
There are no interactions between PEP and different types of food or drinks.
There are no interactions between alcohol, weed or cigarettes and the drugs used in PEP.
It also doesn’t matter if you take PEP with or without food,
30. Do supplements/multivitamins affect PEP?
- Supplements/vitamins containing magnesium, zinc and/or calcium can reduce how some HIV drugs are absorbed. This affect integrase inhibitors: dolutegravir, bictegravir and raltegravir.
- Taking supplements at the opposite end of the day to when you take PEP will avoid this interaction.
- Leave a minimum of 6 hours between PEP and supplements.
- Foods that are fortified have boosted levels of these vitamins will not interact. They will not reduce how much PEP is absorbed.
- A few dosing mistakes are not likely to make any difference to how well PEP works.
- As PEP only lasts four weeks, it is usually better to stop using supplements or drug that might interact if these are not urgently needed. This applies to any supplements that have no interaction data or where the ingredients are not known.
31. Should I continue PEP if I find out my partner is negative?
Some people start PEP when they think their partner was positive.
If they are actually negative, then you no longer need PEP.
But if there is a chance they could have been very recently at risk, then if might be better to still finish PEP. If your partner is still in the window period before they can test positive, their viral load could be very high, making this high risk.
PEP is a way for you to be in control of your health.
This is the reason that after PEP, many people decide to use PrEP.
32. Can I change the bottle used for PrEP?
Yes, changing the container is okay.
Please include everything that comes in the bottle, including anything that helps absorb any moisture.
33. What if I find out my risk partner is HIV negative?
Technically, if your only risk was with someone who is HIV negative, you don’t need to continue PEP.
This involves knowing that this person didn’t have any other recent risks. For example, if they were still in a window period for testing, they might still be HIV positive.
This will depend on their history and on trusting them, so sometimes it might still be better to finish PEP. This way you are still in control of your own health.
34. References and links for further info
Information in this factsheet is based on guidelines from the UK, Europe, South Africa, Australia and the WHO.
- UK PEP prescribing guidelines (2021)
- US CDC information (July 2022)
- US NIH information (February 2024)
- South African Guidelines from Western Cape (2020) PDF
- ASHM. Post-Exposure Prophylaxis after non-occupational and occupational exposure to HIV. Australian National Guidelines (3rd edition). June 2023.
Last updated: 1 November 2024.