Post-exposure prophylaxis (PEP)

Post-exposure prophylaxis (PEP) is a way of preventing HIV infection. It involves using a four-week course of the drugs used to treat HIV, taken very soon after you may have been exposed to the virus.

Decades of experience have shown PEP to be linked with a large reduction in HIV acquisition. However, it is an emergency measure. It shouldn’t be used as a regular method of preventing HIV transmission.

PEP is not needed if you have had sex with someone living with HIV who has an ‘undetectable viral load’. This is when someone’s HIV treatment has reduced the amount of virus in their blood to a very low level. PEP isn’t needed in this case because there is no risk of someone passing HIV on during sex when their viral load is undetectable.

How does PEP work?

PEP works by preventing HIV from entering cells in the body and so stopping someone from getting HIV.

PEP isn’t 100% effective. However, there have been very few reports of HIV infection after the use of PEP. When people taking PEP have acquired HIV, this appears to be linked to:

• not starting PEP quickly enough
• missing doses of PEP or not finishing the 28-day course
• an HIV strain that is resistant to the drugs used in PEP
• having unprotected sex or sharing needles, with further exposure to HIV, while taking PEP.

When must PEP be taken?

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.

It is best to start PEP within 24 hours, but certainly within 72 hours.

How do I access PEP?

Go to a sexual health clinic. If they have an appointments system but they are fully booked, explain that it’s an emergency and that you need to be seen.

If your local sexual health clinic is not open (for example, at the weekend), go to an accident and emergency (A&E) department at a hospital. The staff will contact an HIV specialist who is able to prescribe PEP.

GPs (family doctors) in the UK cannot prescribe PEP.

When you go to get PEP, you will be asked about the sort of sex you have had (or other activity), to work out how high your risk of HIV infection 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. You might also need to have tests for sexually transmitted infections and hepatitis, depending on your situation.

Some people who have had possible exposure to HIV have had difficulty getting PEP. If this happens to you, ask to speak to the on-call HIV doctor, who will know when PEP can be given.

You can also call the THT Direct helpline on 0808 802 1221 for help and advice.

When is PEP recommended?

Expert guidelines in the UK, US, Europe, and from the World Health Organization all  recommend the use of PEP where there is a ‘significant’ risk of HIV infection. Usually this is when:

• the source partner is known to have HIV and a detectable viral load
• the source partner’s HIV status is unknown, but they are from a ‘high risk’ group. For example, this could be a migrant from sub-Saharan Africa, or a man who has sex with men.

Most of the guidelines recommend PEP in the following circumstances:

• Receptive anal sex: If you have had receptive anal sex (when you are the 'bottom') with someone from a high risk group, or someone known to be HIV positive, unless they are on HIV treatment and have an undetectable viral load.
• Insertive anal sex: If you have had insertive anal sex (when you are the 'top') with someone known to be HIV positive, unless the person you had sex with is on HIV treatment and has an undetectable viral load.
• Receptive vaginal sex: If you are a woman who has vaginal sex with a man who is known to be HIV positive, unless he is taking HIV treatment and has an undetectable viral load.
• Work injuries: If you are exposed at work (from a needlestick injury, or if blood or bodily fluids get into your eyes, nose, mouth, or onto broken skin) and the person is known to be HIV positive with an unknown or detectable HIV viral load.
• Drug use: If you have used injecting equipment that’s already been used by someone who is known to be HIV positive, unless they have an undetectable viral load.

Most of the guidelines also consider PEP in the following circumstances:

• Insertive anal sex: If you have had insertive anal sex (when you are the 'top') with someone from a high-risk group whose HIV status is unknown.
• Insertive vaginal sex: If you’re a man who has had vaginal sex with a woman who is known to be HIV positive, unless she is on treatment and has an undetectable viral load.

If you have been sexually assaulted and tell your doctor about it, PEP may be more readily prescribed. They might recommend emergency contraception too, if you need it. Your doctor might also be able to tell you about relevant support centres or voluntary organisations. These organisations can help you report the assault to the police, if this is something you want to do.

You might also be prescribed PEP if you’ve been taking PrEP but have missed doses.

PrEP is a medication taken regularly to prevent HIV. PEP is recommended if there are low PrEP levels in your body and you have condomless sex or share needles with someone known to be HIV positive, unless they have an undetectable viral load.

Doctors will ask you about the doses of PrEP you have taken in the last few days to work out if you need PEP.

When is PEP not recommended?

In some countries, PEP is always recommended if you don’t know the status of the source partner.

However, in the UK, PEP is generally not recommended if you don’t know the HIV status of the person you had sex with (or were assaulted by), except in some cases of anal sex.

Not knowing their HIV status might be worrying for you. However, the reason PEP isn’t recommended is that the number of people in the UK who have HIV and aren’t taking effective treatment is actually quite small.

PEP is not recommended in other circumstances, including:

• oral sex
• semen splashes on skin or in eyes
• human bites
• accidental needle injuries in a public place, such as a park or beach.

Your healthcare team can give you more information about the relative risks of different sexual activities.

Drugs used for PEP

PEP normally consists of three anti-HIV medications, from two different drug classes.

The most recent UK guidelines recommend using a combination tablet combining emtricitabine and tenofovir disoproxil, together with raltegravir (Isentress). You take these medications once a day for at least 28 days.

It is important to take all the doses, at the right time and in the right way, to give PEP the best chance of working. You might be given an extra two days’ drugs if you have been at risk of exposure to HIV in the last 48 hours of the course of PEP.

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.

PEP becomes less likely to work the later you have started it. It’s also less likely to work if you do not take all the doses as prescribed.

Taking PEP at the same time as other drugs can cause drug interactions. When accessing PEP it is important to tell the doctor or pharmacist about anything else you take. This includes over-the-counter medication, vitamins, minerals, herbal remedies and recreational drugs. That way, you can avoid interactions that can result in serious side effects or drugs being ineffective. 

Does PEP cause side effects?

The drugs used in PEP today are less likely to cause side effects than those used in the past. However, all medication can cause side effects, including the HIV medications currently used for PEP.

Side effects are usually worst when you first start taking your medication. For example, you might experience feeling sick, being sick, diarrhoea, or headaches.

An Australian study reported on how many of their patients taking raltegravir and emtricitabine/tenofovir disoproxil as PEP reported side effects. The most common side effects were fatigue (37% of PEP users), diarrhoea (25%), nausea (24%), flatulence (24%), abdominal cramps (21%), bloating (16%), headache (15%), vivid dreams (15%), depression (10%) and thirst (10%).

There’s more information about PEP side effects on another page.

How many times can you take PEP?

It is best not to rely on PEP as a regular way of preventing HIV if you are having condomless sex or sharing drug injecting equipment.

If you have needed PEP more than once in the past, you might want to consider taking PrEP. This is a similar HIV medication that you take regularly to prevent HIV before you’re exposed.

A doctor or nurse at a sexual health clinic can help you think through whether you need PrEP and whether it is suitable for you. This will involve being asked about the type of sex you have been having and expect to have in the future.

PEP and pregnancy

If you’re able to become pregnant but don’t want to be and you’ve had condomless sex without any other form of contraception, you may want to consider emergency contraception.

You can access the emergency contraceptive pill from pharmacies. It is usually also available from GPs, sexual health clinics and A&E departments.

However, it is important to let the doctor or pharmacist know if you are taking PEP. Some anti-HIV medication can affect the way the emergency contraceptive pill works, and you will need to take an increased dose.

If you are pregnant, you can still take PEP. Your doctor may ask you to take a different dose of PEP, twice a day, or you may be prescribed an alternative medication.

If you are breastfeeding, you can still take PEP. Doctors will choose a regimen that is suitable for you and your baby (who might absorb the drug via breast milk) and offer you advice.