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Post-exposure prophylaxis (PEP)

Michael Carter, Greta Hughson
Published: 25 September 2012

If you have had unprotected sex (including if a condom broke or came off during sex) or shared drug injecting equipment with somebody you know is HIV positive, or is from a group at high risk of HIV, such as gay men or African migrants, then you may be able to access a short course of HIV treatment intended to reduce the risk of you becoming infected with HIV.

This is called post-exposure prophylaxis, or PEP for short.

The professional organisation of the UK's sexual health doctors has 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 used on people who were known to be HIV-positive or at risk of HIV.

PEP is not a ‘cure’ for HIV. PEP may prevent HIV from entering cells in the body and so stop you from becoming infected with 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 and certainly within 72 hours.

Medicine used for PEP

PEP normally consists of three anti-HIV drugs, from two of the different classes, usually two from the NRTI class, plus a boosted protease inhibitor.

PEP should be taken for a month, and it is important to take all the doses, at the right time and in the right way.

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 the accident and emergency department, who will contact an HIV specialist who is able to prescribe PEP.

Guidelines for prescribing PEP

There are guidelines stating when PEP should be considered. The guidelines 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.

In the UK, the guidelines were updated in 2011, to 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 'undetectable viral load' because it is below the limit of detection on standard tests), then it is very unlikely that HIV would be passed on during sex and PEP would not be recommended.

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 becoming infected 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 sooner PEP is accessed the better. The guidelines state that it should be provided within 72 hours of the possible HIV exposure.

PEP is considered to be emergency treatment. You should be able to get it free of charge whatever your immigration status is.

Despite these guidelines, some people who have had possible HIV exposure, including gay men, have had difficulty getting PEP. In these circumstances, you may find this information from the CHAPS HIV prevention partnership helpful:

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 an African migrant or a gay man.
  • 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 if you have had vaginal sex with someone who is known to be HIV positive, unless they have an undetectable viral load.
  • Sharing injection equipment: PEP is recommended if you have shared injecting equipment with someone who is known to be HIV positive, unless they have an undetectable viral load.

In other circumstances, including oral sex when a man who is known to be HIV positive has ejaculated into your mouth, PEP would be considered if there are other factors involved that could increase the risk of transmission. These factors include the presence of another sexually transmitted infection or sexual assault.

Talk to a health professional for more details. You can download the guideline document at


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

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.

If you are already HIV positive, but don’t know it, there is a chance of developing drug resistance when you take PEP if you don’t take your doses properly. This could limit your treatment options in the future.

Other things to consider

PEP is not 100% effective, so it would make good sense not to rely on access to PEP if you are having unprotected sex or sharing drug injecting equipment. Condoms, when used properly, are an effective way of preventing the spread of HIV and most other sexually transmitted infections. PEP won’t stop you becoming infected while you’re taking it, so it’s sensible to use condoms during that period as well.

If you are a woman and not using other forms of contraception, then if you have had unprotected sex you may also wish to consider emergency contraception. 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 on HIV treatment, 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.

Post-exposure prophylaxis (PEP)

Contact NAM to find out more about the scientific research and information used to produce this factsheet.

This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.
Community Consensus Statement on Access to HIV Treatment and its Use for Prevention

Together, we can make it happen

We can end HIV soon if people have equal access to HIV drugs as treatment and as PrEP, and have free choice over whether to take them.

Launched today, the Community Consensus Statement is a basic set of principles aimed at making sure that happens.

The Community Consensus Statement is a joint initiative of AVAC, EATG, MSMGF, GNP+, HIV i-Base, the International HIV/AIDS Alliance, ITPC and NAM/aidsmap