If a person is exposed to HIV during sex, many sexual health (GUM) and HIV clinics can provide them with a short course of anti-HIV drugs to try to prevent infection, if there has been significant risk. This is called post-exposure prophylaxis, or PEP for short, and it is becoming more widely available. PEP is not thought to be 100% effective and may have side-effects.

PEP may also be considered in cases of rape and sexual assault where there is thought to have been a risk of HIV transmission.

It is important to get and take PEP as soon as possible after potential exposure to HIV – ideally within four hours, and certainly within 72 hours.

If you are taking anti-HIV drugs and have unprotected sex with a person who is HIV-negative or whose HIV status you do not know, or if there is a condom accident during sex, you may be tempted to offer them some of your anti-HIV drugs in an attempt to reduce the risk of them becoming infected with HIV.

This is not a good idea. Some anti-HIV drugs work better as PEP than others, and a full PEP course should last a month. It could be risky as some HIV drugs, particularly abacavir (Ziagen), nevirapine (Viramune) and etravirine (Intelence), can cause an allergic reaction or severe side-effects that can be fatal. There is also a chance that the person you are giving your HIV drugs to could already be infected with HIV and not know it. In this case, taking a few doses of your HIV medicine could give the HIV in their body a chance to develop resistance to those drugs. This would limit their future treatment options.

The thought that you may have exposed somebody to the risk of HIV infection may be worrying. If you do think that PEP might be appropriate, encourage them to go to their local sexual health clinic as soon as possible. If it is closed, they should go to the accident and emergency department of their local hospital and ask for PEP. Staff there should contact the on-call HIV doctor.

Studies are underway to see if taking HIV treatment before risky sex can stop an HIV-negative person becoming infected with HIV.

The use of anti-HIV drugs in this way is called pre-exposure prophylaxis (PrEP). It is not yet certain if PrEP is effective and safe and at the moment it’s only being provided in clinical trials. For the same reasons as PEP, it’s important that you don’t give an HIV-negative partner your anti-HIV drugs.