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Medically Reviewed by:

Dr Danae Maragouthakis

Post-exposure prophylaxis (PEP) is a treatment that can prevent human immunodeficiency virus (HIV) infection after actual or potential exposure to the virus. PEP is most often used by people who have had unprotected intercourse, where there is a risk of contracting HIV. PEP is also used by healthcare workers who have potentially been exposed to HIV, for example, after accidentally pricking themselves with an infected needle.

For the treatment to be effective, it must be taken within 72 hours after exposure. In fact, according to a UK study, having 5-day starter parks of PEP at home for people at risk of having HIV resulted in a significantly shorter time to start the drug, which is the most important factor for PEP efficacy.

Although PEP can prevent HIV infection, it is important to note that it is an emergency measure, which should not be relied upon as a way to prevent HIV transmission.

When Would I Need PEP?

If you have been possibly exposed to HIV, it is important to attend Accident and Emergency (A&E), your general practitioner (GP) or a sexual health service right away to talk about if you need PEP. You might need PEP if you have potentially been exposed to HIV:

  • During sex
  • Through sharing needles or syringes
  • If you have been sexually assaulted

If you have potentially been exposed to HIV in any of these ways, a healthcare professional will assess your risk, and may offer you PEP. If PEP is appropriate for you and you choose to take this, you will be given a supply of pills to take daily for 28 days. You will require blood testing before starting PEP and you will be advised to return to your healthcare provider after the course of treatment for further testing.

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Where Do I Get PEP?

PEP is available from A&E, sexual health clinics, and some GP practices. If you cannot get to a sexual health clinic or GP practice right away, go to A&E as soon as you can. PEP is most effective when taken as soon as possible.

When you ask for PEP, a healthcare provider will talk to you about what happened to decide if PEP is right for you. If PEP is suitable for you, they will take your blood for HIV testing, and will offer you testing for other sexually transmitted infections (STIs). If you already have HIV, you will not be able to use PEP.

How to Take PEP

The standard course of PEP involves taking medication twice per day for 28 days. It is vital that you take the medication exactly as directed and complete the entire course. You should not skip doses, as this can reduce the efficacy of the drug. It is also important to attend any follow-up appointments.

PEP can be taken with or without food, and tablets should not be crushed unless advised by a healthcare professional. Common side effects include nausea, headaches, stomach upset and tiredness, which are more common in the first few days of taking the medication. If you struggle with side effects, speak with your healthcare provider.

How Effective is PEP?

Use of PEP following exposure to HIV is a highly effective method of preventing acquisition of the virus. Most evidence comes from animal studies and studies in healthcare workers.

One of the first studies, in 1997, showed that PEP treatment in healthcare workers exposed to HIV reduced transmission by 80% compared to exposed workers who did not take PEP. More recent data from Public Health England from 2004–2013 looking at healthcare workers who had been exposed to body fluids, most of which started HIV PEP, have revealed no new cases of HIV. In fact, there has only been one documented case of HIV acquisition following occupational exposure in a healthcare worker in the UK since 1997, suggesting that PEP is highly effective.

In a 2005 study of people who reported HIV exposure either through sex or injecting drug use, and who took a 28-day course of PEP, started within 72 hours, only 1% were found to have HIV after 12 weeks, suggesting PEP is also highly effective at preventing HIV transmission following exposure via sex.

Studies support initiating PEP soon after exposure to HIV (ideally within 24 hours) and continuing the course for 28 days reduces the risk of acquiring HIV infection after exposure. PEP is not recommended if exposure was more than 72 hours ago.

Based on the evidence, one can conclude that PEP, if taken at the recommended dosage over a certain number of days (28), can be highly effective in preventing HIV infection.

What If I Can’t Get PEP In Time?

Timing is really important when it comes to getting PEP. The longer you wait, the greater the chance PEP won’t work. However, if you are unable to get PEP within 72 hours, it is still important to seek advice from a healthcare professional.

You may be offered HIV testing soon after your potential exposure, as early treatment of the infection is paramount.

What’s the Difference Between PEP and PrEP?

PEP stands for post-exposure prophylaxis, while PrEP stands for pre-exposure prophylaxis. Therefore, PEP is given after exposure to HIV, while PrEP is prescribed to prevent HIV before a person is exposed. So, the key difference between PEP and PrEP is timing.

PrEP is for people who do not have HIV, but may be at risk of getting it. This may include those who:

  • Have an HIV-positive partner
  • Have multiple partners
  • Have a partner whose HIV status is unknown
  • Share needles or other equipment to inject drugs
  • Have contracted an STI in the past 6 months
  • Have not consistently used a condom

Stay On Top of Your Sexual Health

If you feel that you have been exposed to HIV and are worried about contracting the virus, get PEP as soon as possible. When it comes to your sexual health, there are many things you can do to stay safe and protected, and this is one.

Never think it is too late to get treatment or practice prevention. If you are worried about your sexual health, you can always take an at-home test kit to test for an STI like HIV. But beyond that, we always recommend getting support and advice to protect your mental, physical, and sexual well-being. Have any questions? Contact us today. We’re always happy to help!


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Yoxly's Awesome Contributors (YACs) are a diverse group of individuals who are passionate about public health, and committed to furthering our mission. Yoxly provides a platform where a variety of sexual health topics (some more awkward than others!) can be explored, in an informative and non-judgmental way. If you'd like to become one of Yoxly's Awesome Contributors, contact us!