PEP and PrEP


Post exposure prophylaxis (PEP) is anti-HIV medication that is prescribed to a HIV negative person, after a potential exposure to HIV, to protect them from being infected with HIV. It should be started within 72 hours of the exposure, the earlierthe better. It won’t usually be prescribed after 72 hours. PEP needs to be taken daily for 28 days.

Evidence suggests PEP reduces the chances of HIV infection by approximately 80%. It is not a cure for HIV and it doesn’t work in all cases in preventing HIV transmission: some strains of HIV aren't affected by the medicine. Also if you don’t takeit correctly or start it too late it may fail to protect you.

PEP can be provided in A and E departments or sexual health services.

It can have some side effects, such as tiredness, sickness, diarrhoea, and headache.

PEP may be considered or recommended in the following circumstances

1. If your partner is HIV positive AND

  • you had unprotected anal sex with them OR
  • you had unprotected vaginal sex with them OR
  • you shared their equipment for injecting drugs

2. Your partner’s HIV status is unknown or unclear, but they belong to a group where the rates of HIV are high. Such high risk groups include gay or bisexual men; those who have migrated to the UK from countries with high HIV prevalence (ie >1%), eg SubSaharan Africa; Injecting drug users from high risk countries such as Eastern Europe and Central Asia AND

  • you had unprotected anal sex with them OR
  • you had unprotected vaginal sex with them OR
  • you shared their equipment for injecting drugs

If you have an HIV positive partner and they have been on HIV treatment for more than 6 months and they have been taking it properly with an undetectable viral load you probably won’t need PEP. In this situation the risk of catching HIV (if theyare your only partner) is near to zero.

The following risks do not usually warrant PEP: human bites, semen splash to the eye, oral sex.

But… there are many factors that are involved in deciding whether you require PEP after an exposure or not so if in doubt attend A and E or a sexual health clinic as soon as possible.

Before you are given PEP you need to have a HIV test. This is because if you are already HIV positive we need to do further tests to rule out drug resistance, before starting treatment on you.

You will need to have further HIV tests after completing the PEP course to confirm you have not been infected. It is also recommended you have testing for other blood borne infections like hepatitis B and hepatitis C as well as a full sexual transmittedinfection screen (chlamydia, syphilis and gonorrhoea).

Additional Links


Pre-exposure prophylaxis (PrEP) is a course of HIV drugs taken by an HIV negative person to lower the chance of becoming infected with HIV in the future. There are different ways you can take PrEP. You can take it every day for continuous protection.Alternatively, if you can predict your risk in advance you can use the ‘event based’ option.

The advice is based on two studies (Ipergay and PROUD) which looked at different ways of taking the treatment. Both studies showed an 86% reduction in catching HIV. In PROUD study (done in the UK) people took PrEP every day. In the Ipergay studythe person took PrEP around an “event” – they took two PrEP pills between 2-24 hours before sex, then one PrEP pill every day until two days had passed after the last unprotected sex. The Ipergay regimen is not recommended for vaginalsex. No one in either study who took PrEP as directed became HIV positive.

Before and every three months whilst you are taking PrEP you need to attend a sexual health clinic for baseline and regular HIV tests, checks on your kidney, your hepatitis status check and full sexual health screening.

You can get PrEP via

  • The PrEP Impact study – this offers NHS funded PrEP to people at high risk of catching HIV.
  • You can buy PrEP online for your own use.

Some scenarios where you might consider starting on PrEP include

  • You are a gay or bisexual man and have recently had unprotected anal sex and are likely to have it again in the next few months
  • You have an HIV positive partner who is not on treatment and you are likely to have unprotected sex with that person.
  • You have recently had a rectal STI (gonorrhoea, LGV or chlamydia in your anus) or syphilis.
  • You have used PEP (post exposure prophylaxis) in the last few months
  • You participate in chemsex parties

PrEP is not recommended if you are already HIV positive. Nor is it recommended if you have an HIV positive partner who has been taking treatment for a while and has an undetectable viral load. In this situation the risk of catching HIV (if theyare your only partner) is near to zero.

Additional Links

Sexual Health London offers a free STI testing across most of London.
Simply register with SHL and complete an online consultation to order your free STI test kit.