If you believe you may have recently been exposed to HIV, you should consider post-exposure prophylaxis (PEP). In the UK, PEP is now generally available to people if they ask for it no more than three days after a possible exposure to HIV.

PEP is a month-long course of HIV treatment that may stop HIV infection after someone has been exposed to the virus.

To be prescribed PEP you should go to a sexual health (or GUM) clinic as soon as possible, or, if it is outside their opening hours, you should go to the nearest Accident and Emergency department (A&E).

The sooner you take PEP after a possible exposure to HIV the more likely it is to be effective. Studies suggest that it may be close to 100% effective if taken within 24 hours, but no more than 50% effective if taken more than 72 hours after an exposure to HIV and very much less effective if taken later than this.

PEP, if prescribed and taken properly, may be able to block HIV infection completely if treatment lasts one month, though there are cases of ‘breakthrough’ infections in which HIV became established despite PEP. Treatment does not need to continue for longer than one month, because the aim of treatment is to prevent HIV from entering cells in the body and establishing an infection.

PEP has been used for some time in cases where healthcare workers were exposed to HIV during medical procedures. More recently, it has been available for some people who may have been exposed to HIV during sex. In April 2006, the Chief Medical Officer for England, Sir Liam Donaldson, wrote to all NHS trusts emphasising that PEP should be made available.

Clinics will not automatically offer PEP to everyone who believes they have been at risk of HIV infection. They will use a number of questions to decide whether you have been at high risk of infection. These may include:

• Was your partner known to be HIV-positive?
• Was your partner in a high-risk group?
• What kind of sex did you have: anal, vaginal, or oral?
• Did ejaculation occur into your body, or were you the active partner?
• Did you inject blood into your veins if you were sharing needles?
• Was sexual intercourse violent, traumatic or due to sexual assault?
• Where did your partner come from - a metropolitan area with high HIV prevalence, or a small town with low prevalence?
• Are you able to adhere to a four-week course of treatment that might produce unpleasant side-effects?

They will make a decision about whether you need PEP based on the information you give them, so it is important to be honest about what happened.

If you are having problems accessing PEP, the Terrence Higgins Trust website has a copy of the letter from Sir Liam Donaldson (mentioned above) that can be downloaded and printed, so you can take it with you when you seek help. There is also a self-assessment to help you work out whether PEP would be recommended in your situation or not. Click here for the Terrence Higgins Trust PEP pages.