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.

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.

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

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.

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) 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. 



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

sexually transmitted infections (STIs)

Although HIV can be sexually transmitted, the term is most often used to refer to chlamydia, gonorrhoea, syphilis, herpes, scabies, trichomonas vaginalis, etc.

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 supresses 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.

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, including gay men, 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.
  • 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. It 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.

PEP may be considered if you have given oral sex to a man known to be HIV positive, who has ejaculated into your mouth, only if your mouth is injured or he has a very high viral load. PEP is not recommended in other circumstances, including cunnilingus (oral sex performed on a woman’s genitalia), 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 a 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 Truvada (a fixed-dose combination tablet combining emtricitabine and tenofovir) from the NRTI class, and raltegravir (Isentress) from the integrase inhibitor class. There are alternative drugs available if there is a reason you cannot take either of those.

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 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.


HIV treatment 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, tiredness, and generally feeling unwell. The drugs used in a course of PEP today are less likely to cause side-effects than those used in the past.

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.

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.

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