If you have had unprotected 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, 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.

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.

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. However, the guidelines state that PEP may still be considered after this point for possible HIV exposure in high-risk groups.

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.

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

There are guidelines stating when PEP should be considered. It is recommended if you have had unprotected insertive or receptive anal, or vaginal sex with someone who is known to be HIV-positive.

It is also recommended if you have had unprotected receptive anal sex with someone whose HIV status is not known.

It may be considered if you have had insertive anal sex, insertive or receptive vaginal sex, or fellatio (performing oral sex on a man) with ejaculation with someone who is known to be HIV positive or whose HIV status is unknown. It may also be considered for semen splashes in the eye if a person is known to be HIV-positive.

When you go to get PEP, you will be asked about the sort of unsafe 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, but that it may be provided after this time for high-risk groups.

PEP is considered to be a life-saving 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: http://www.pep.chapsonline.org.uk/pep_availability.htm

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.

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 inject drugs, not sharing needles or other injecting equipment will help prevent the transmission of HIV and other infections.