Post-exposure prophylaxis (PEP)

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Key points

  • PEP involves taking a 28-day course of anti-HIV drugs, after possible exposure to HIV.
  • Doctors will assess your risk of HIV infection before prescribing PEP.
  • PEP is available from sexual health clinics and accident & emergency departments.

Post-exposure prophylaxis, or PEP, is a method of preventing HIV infection. It involves using a four-week course of the drugs used to treat HIV, taken very soon after a person may have been exposed to the virus.

Decades of experience has shown PEP to be associated with a substantial reduction in HIV acquisition. However, it is an emergency measure, rather than one to be used as a regular method of preventing HIV transmission.

The professional organisation of the UK's sexual health doctors produces guidelines recommending the circumstances in which PEP should be used.

Purpose of PEP

PEP has been used for many years for healthcare workers who have had possible exposure to HIV; for example, after accidentally pricking themselves with needles previously used on people who were known to have HIV or considered at high risk of having HIV.

In the UK, PEP has also been made available to people who may have been exposed to HIV in other ways, including during sex.

PEP is not a ‘cure’ for HIV. PEP may prevent HIV from entering cells in the body and so stop someone from getting HIV. PEP isn’t 100% effective. However, there have been very few reports of HIV infection after the use of PEP.

When people taking PEP have acquired HIV, this appears to be related to:

  • not starting PEP promptly
  • missing doses of PEP or not finishing the 28-day course
  • an HIV strain that is resistant to the drugs used in PEP
  • having unprotected sex or sharing needles, with further exposure to HIV, while taking PEP.

Timing of PEP

To have the best chance of it being effective, you need to start taking PEP as soon as possible after the possible exposure to HIV.

It is best to start PEP within 24 hours, but certainly within 72 hours.

Where to get PEP

Go to a sexual health clinic. If they operate an appointments system and are fully booked, explain that it’s an emergency and that you need to be seen. If your local sexual health clinic is not open (for example, at the weekend), go to an accident and emergency (A&E) department at a hospital, where staff will contact an HIV specialist able to prescribe PEP. GPs (family doctors) in the UK cannot prescribe PEP.

Guidelines for prescribing PEP

In the UK, HIV and sexual health doctors have produced guidelines setting out when PEP may be an option to prevent sexual transmission of HIV. These take into account the type of sex you had and also what is known about the 'source partner', e.g. the person who has HIV or might have HIV. PEP may also be used when you have used injecting equipment previously used by someone who has, or may have, HIV.


post-exposure prophylaxis (PEP)

A month-long course of antiretroviral medicines taken after exposure or possible exposure to HIV, to reduce the risk of acquiring HIV.

undetectable viral load

A level of viral load that is too low to be picked up by the particular viral load test being used or below an agreed threshold (such as 50 copies/ml or 200 copies/ml). An undetectable viral load is the first goal of antiretroviral therapy.

viral load

Measurement of the amount of virus in a blood sample, reported as number of HIV RNA copies per milliliter of blood plasma. Viral load is an important indicator of HIV progression and of how well treatment is working. 


pre-exposure prophylaxis (PrEP)

Antiretroviral drugs used by a person who does not have HIV to be taken before possible exposure to HIV in order to reduce the risk of acquiring HIV infection. PrEP may either be taken daily or according to an ‘event based’ or ‘on demand’ regimen. 


How well something works (in real life conditions). See also 'efficacy'.

These guidelines take into account the viral load of the person with HIV, if this is known. If someone with HIV is taking HIV treatment and it suppresses their viral load to a very low level (referred to as an undetectable viral load because it is below the limit of detection on standard tests), there is no risk of passing HIV on during sex. PEP is not recommended in this situation.

When you go to get PEP, you will be asked about the sort of sex (or other activity) you have had, to assess how high your risk of HIV infection is. You will need to have an HIV test to check you don’t already have HIV. You will also need to agree to be tested again when you have finished the course of PEP, when some additional tests for sexually transmitted infections and hepatitis might be required, depending on your circumstances.

The guidelines recommend the use of PEP where there is a ‘significant’ risk of HIV infection. Despite these guidelines, some people who have had possible HIV exposure have had difficulty getting PEP. If this happens to you, ask to speak to the on-call HIV doctor, who will know when PEP can be given. You can call the THT Direct helpline on 0808 802 1221 for help and advice.

When is PEP recommended?

  • Receptive anal sex: PEP is recommended if you have had receptive anal sex (when you are the 'bottom') with someone who is known to be HIV positive or who is thought to be from a high-prevalence country or risk group, e.g. from sub-Saharan Africa or a man who has sex with men (MSM). The exception to this is if the person you had sex with is known to be on HIV treatment and to have an undetectable viral load.
  • Insertive anal sex: PEP is recommended if you have had insertive anal sex (when you are the 'top') with someone who is known to be HIV positive, unless they have an undetectable viral load.
  • Receptive vaginal sex: PEP is recommended for women who have had vaginal sex with a man who is known to be HIV positive, unless he has an undetectable viral load.
  • Insertive vaginal sex: PEP will be considered for men who have had vaginal sex with a woman who is known to be HIV positive, unless she has an undetectable viral load.
  • Non-sterile injection equipment: PEP is recommended if you have used injecting equipment previously used by someone who is known to be HIV positive, unless they have an undetectable viral load.
  • Sexual assault: If you have been sexually assaulted and tell your doctor about it, PEP may be more readily prescribed. They might also recommend emergency contraception and could signpost you to support centres or voluntary sector organisations, who can guide you in the process of reporting the assault to the police, if this is something you want to do.
  • Missed PrEP doses: PrEP is a similar HIV medication taken regularly to prevent HIV. If you have been taking PrEP but missed doses in the days before risky sex, PEP might be prescribed. Specifically, PEP is recommended if there are low PrEP levels in your body and you have had condomless sex or share needles with someone known to be HIV positive, unless they have an undetectable viral load. Doctors will ask you about the doses of PrEP you have taken in the last few days to work out if PEP is needed. 

PEP is generally not recommended if you don’t know the HIV status of partner, except some cases of anal sex. Not knowing your partner’s HIV status might be worrying you, but PEP isn’t recommended because the number of people in the UK who have HIV but aren’t taking effective treatment is actually quite small. 

PEP is not recommended in other circumstances, including oral sex, semen splashes on skin or in eyes, human bites, or a needlestick injury in the community.

Your healthcare team can give you more information about the relative risks of different sexual activities. You can also assess the risk of HIV transmission using this tool on the Terrence Higgins Trust website.

Drugs used for PEP

PEP normally consists of three anti-HIV drugs, from two of the different classes. The most recent UK guidelines recommend using a fixed-dose combination tablet combining emtricitabine and tenofovir disoproxil from the NRTI class, together with raltegravir (Isentress) from the integrase inhibitor class.

PEP should be taken for 28 days. It is important to take all the doses, at the right time and in the right way, to give PEP the best chance of working. You might be given an additional two days’ drugs if you have been at risk of exposure to HIV in the last 48 hours of the course of PEP.

If you have been exposed to a strain of HIV that is resistant to some anti-HIV drugs then it’s possible that PEP won’t work. PEP is less likely to work the later you have started it, or if you do not take all the doses.

Side effects

HIV medications can cause side effects which tend to be worst when you first start taking them. If you are taking PEP you could experience some unpleasant side effects such as feeling sick, being sickdiarrhoea, and generally feeling unwell.

An Australian study reported on how many of their patients taking raltegravir and emtricitabine/tenofovir disoproxil as PEP reported side effects. The most common side effects were fatigue (37% of PEP users), diarrhoea (25%), nausea (24%), flatulence (24%), abdominal cramps (21%), bloating (16%), headache (15%), vivid dreams (15%), depression (10%) and thirst (10%).

Nonetheless, the drugs used in a course of PEP today are less likely to cause side effects than those used in the past, making them more tolerable and it easier to complete the 28-day course.

Other things to consider

It is best not to rely on PEP as a regular way of preventing HIV if you are having condomless sex or sharing drug injecting equipment. Condoms, when used properly, are an effective way of preventing HIV and most other sexually transmitted infections (STIs). PEP won’t stop you getting other STIs while you’re taking it, so it’s sensible to use condoms during that period as well. Staff at sexual health clinics can provide information and advice about sexual health and how best to protect yourself from HIV and other STIs.

"Guidelines have set out the range of activities where risk of HIV is great enough that PEP is recommended."

If you have needed PEP more than once in the past, you might want to consider taking PrEP (a similar HIV medication taken regularly to prevent HIV before exposure). A doctor or nurse at a sexual health clinic can help you think through whether you need PrEP and whether it is suitable for you. This will involve being asked about the type of sex you have been having and expect to have in the future.

If you are a woman, have had sex without a condom and did not use any form of contraception, you may want to consider emergency contraception if you do not wish to be pregnant. You can buy the emergency contraceptive pill from chemists, and it is usually also available from GPs, sexual health clinics and A&E departments. However, it is important to let the doctor or pharmacist know if you are taking PEP, as some anti-HIV drugs can interfere with the way the emergency contraceptive pill works, and you will need to take an increased dose. As with PEP, you need to take the pill within 72 hours of having sex, and ideally sooner. An alternative and very effective method of emergency contraception is to have an intrauterine device (IUD) fitted. Talk to your doctor about the best option for you.

Taking PEP at the same time as other drugs can produce drug interactions. When accessing PEP it is important to tell the doctor or pharmacist about any other medicines you take. This includes over-the-counter medication, vitamins/minerals, herbal remedies and recreational drugs. That way, you can avoid interactions that can result in serious side effects or drugs being ineffective.  

If you are pregnant, you can still take PEP. In the UK, raltegravir and emtricitabine/tenofovir disoproxil is the regimen usually used during pregnancy, however, doctors will choose alternative drugs if there is a reason you cannot take those.

If you are breastfeeding, you can still take PEP. Doctors will choose a regimen that is suitable for you and your baby (who might absorb the drug via the breast milk) and offer appropriate advice.

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