Q and A

PEP: your questions answered…

The information below will answer most questions about PEP.

Please read this before sending a question to the Q&A service.

Please also remember that the risk of catching HIV from one time is usually very low.

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.

It is only effective against HIV.

PEP does NOT prevent other infections. It is NOT a contraceptive.

Does PEP work?

Yes, PEP can stop infection. It also reduces the risk.

Starting early gives PEP the best chance of working.

Taking the full course as prescribed is also important.

How does PEP work?

HIV infection takes a little time to develop.

This leaves a short window for HIV meds to work, if taken soon after any risk.

Having good levels of HIV drugs at the right time can often stop infection.

How well does PEP work?

PEP is very effective but there are limited results on how well it works.

This is because it is a very difficult thing to study. Firstly, HIV is quite a difficult virus to catch. Even after higher-risk sex, this risk is probably 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.

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 or 48 hours to start.
  • The earlier it is started the earlier it can start to work.
  • There is no benefit from starting after more than 72 hours (3 days).

How many drugs are in PEP?

PEP uses three drugs.

  • Sometimes all three drugs are in the same pill.
  • Sometimes PEP involves taking several pills every day.
  • Only using two drugs will not be as effective as three. Sometimes though, if you only have two drugs, it is better than none.

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.

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.

What if I am late or if I miss a dose?

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.

  • Do not double the next dose, this will not help.
  • A single missed dose will be okay, it is not likely to make a big difference.
  • Still continue to finish the course of PEP.

Can I stop earlier than 28 days?

PEP is given for 28 days.

  • If you stop earlier, it might not be as effective.
  • 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.

What if I get symptoms 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.

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.

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. Seroconversion is often like a very heavy flu that lasts for about a week and then resolves.
  • 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.
  • Even if the symptoms are more serious, this does not mean it is HIV. It might, for example, be COVID. Please talk to a doctor if any symptoms are serious.

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 after eight weeks.

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.

Please see:
What is seroconversion and what are the symptoms?

When is PEP used?

PEP is only used if you had penetrative sex.

  1. With someone whose HIV status you don’t know, or
  2. 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.

Which drugs are used for PEP

In the UK, PEP is with tenofovir/emtricitabine + raltegravir.

  • These are taken once a day, at roughly the same time every day for 28 days.
  • PEP can be taken with or without food.

Different drugs can be used depending on where you live.

Can efavirenz be used in PEP?

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.

This PEP would still work against HIV, but the side effects might make your quality of life very difficult during this month.

Even so, we still hear from people in different countries where efavirenz is still included in PEP.

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 before finding out the result.

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 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.

If you are negative after six weeks you do not have to take a second test.

Can I have sex when on PEP?

Please take a break from sex while taking PEP.

  • This is because if you were at risk for HIV you were also at risk from 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.
  • PEP does not need to contain the same medications as PrEP to be successful at preventing second exposures.
  • 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.

Can I masturbate when on PEP?

Yes, you can do this as much as you like.

It will not have any effect on PEP.

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.

Please see these links for information on PrEP.

If I do have sex on PEP, does my partner need PEP too?


This is because the PEP will mean you are either negative or have a very low viral load.

What about PEP 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.

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.

How can I get PEP?

PEP can only be prescribed by a doctor.

In the UK PEP is available from:

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.

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.

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.

Does alcohol affect PEP?

There are no interactions between alcohol and the drugs used in PEP.

Does supplements/multivitamins affect PEP?

  • Supplements/vitamins containing magnesium, zinc and/or calcium can reduce how much PEP is absorbed.
  • Taking supplements at the opposite end of the day to when you take PEP will avoid this interaction.
  • Have 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.


UK PEP prescribing guidelines (2021)
https://onlinelibrary.wiley.com/doi/10.1111/hiv.13208 (web page)
https://www.bashhguidelines.org/media/1269/pep-2021.pdf (PDF)

US CDC information (2021)

US NIH information (2021)

South African Guidelines – Western Cape (2020)

Last updated: 1 April 2023.