Post-exposure prophylaxis, or PEP, is a method of preventing HIV infection. It involves using a short course of the drugs used to treat HIV, taken very soon after a person may have been exposed to the virus.

It is an emergency measure, rather than one to be used as a regular method of preventing HIV transmission.

The professional organisation of the UK's sexual health doctors produces guidelines recommending the circumstances in which PEP should be used.

Purpose of PEP

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 previously used on people who were known to have HIV or considered at high risk of having HIV.

In the UK, PEP has also been made available to people who may have been exposed to HIV in other ways, including during sex.

"Guidelines have set out the range of activities where risk of HIV is great enough that PEP is recommended."

PEP is not a ‘cure’ for HIV. PEP may prevent HIV from entering cells in the body and so stop someone from getting HIV. PEP isn’t 100% effective. However, there have been very few reports of HIV infection after the use of PEP.

Timing 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.

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

Where to get PEP

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 your local sexual health clinic is not open (for example, at the weekend), go to an accident and emergency department at a hospital, where staff will contact an HIV specialist able to prescribe PEP. GPs (family doctors) cannot prescribe PEP.

Guidelines for prescribing PEP

In the UK, HIV and sexual health doctors have produced guidelines setting out when PEP may be an option to prevent sexual transmission of HIV. These take into account the type of sex you had and also what is known about the 'source partner', e.g. the person who has HIV or might have HIV. PEP may also be used when you have used injecting equipment previously used by someone who has, or may have, HIV.

These guidelines take into account the viral load of the person with HIV, if this is known. If someone with HIV is taking HIV treatment and it supresses their viral load to a very low level (referred to as an undetectable viral load because it is below the limit of detection on standard tests), then it is very unlikely that HIV would be passed on during sex. PEP is not recommended in this situation.

When you go to get PEP, you will be asked about the sort of sex (or other activity) you have had, to assess 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.

The guidelines recommend the use of PEP where there is a ‘significant’ risk of HIV infection. Despite these guidelines, some people who have had possible HIV exposure, including gay men, 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 call the THT Direct helpline on 0808 802 1221 for help and advice.

When is PEP recommended?

  • Receptive anal sex: PEP is recommended if you have had receptive anal sex (when you are the 'bottom') with someone who is known to be HIV positive or who is thought to be from a high-prevalence country or risk group, e.g. from sub-Saharan Africa or a man who has sex with men (MSM). The exception to this is if the person you had sex with is known to be on HIV treatment and to have an undetectable viral load.

  • Insertive anal sex: PEP is recommended if you have had insertive anal sex (when you are the 'top') with someone who is known to be HIV positive, unless they have an undetectable viral load.

  • Vaginal sex: PEP is recommended for women who have had vaginal sex with a man who is known to be HIV positive, unless he has an undetectable viral load. It will be considered for men who have had vaginal sex with a woman who is known to be HIV positive, unless she has an undetectable viral load.

  • Non-sterile injection equipment: PEP is recommended if you have used injecting equipment previously used by someone who is known to be HIV positive, unless they have an undetectable viral load.

PEP may be considered if you have given oral sex to a man known to be HIV positive, who has ejaculated into your mouth, only if your mouth is injured or he has a very high viral load. PEP is not recommended in other circumstances, including cunnilingus (oral sex performed on a woman’s genitalia), semen splashes on skin or in eyes, human bites, or a needlestick injury in the community.

Your healthcare team can give you more information about the relative risks of different sexual activities. You can also assess the risk of HIV transmission using a tool on the Terrence Higgins Trust website.

Drugs used for PEP

PEP normally consists of three anti-HIV drugs, from two of the different classes. The most recent UK guidelines recommend using Truvada (a fixed-dose combination tablet combining emtricitabine and tenofovir) from the NRTI class, and raltegravir (Isentress) from the integrase inhibitor class. There are alternative drugs available if there is a reason you cannot take either of those.

PEP should be taken for 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 additional two days’ drugs if you have been at risk of exposure 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 is less likely to work the later you have started it, or if you do not take all the doses.


HIV treatment can cause side-effects which tend to be worst when you first start taking them. If you are taking PEP you could experience some unpleasant side-effects such as feeling sick, being sickdiarrhoea, tiredness, and generally feeling unwell. The drugs used in a course of PEP today are less likely to cause side-effects than those used in the past.

Other things to consider

It is best not to rely on PEP as a regular way of preventing HIV if you are having unprotected sex or sharing drug injecting equipment. Condoms, when used properly, are an effective way of preventing HIV and most other sexually transmitted infections. PEP won’t stop you getting other STIs while you’re taking it, so it’s sensible to use condoms during that period as well. Staff at sexual health clinics can provide information and advice about sexual health and how best to protect yourself from HIV and other STIs.

If you are a woman, have had sex without a condom and did not use any form of contraception, you may want to consider emergency contraception if you do not wish to be pregnant. You can buy the emergency contraceptive pill from chemists, and 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, as some anti-HIV drugs can interfere with the way the emergency contraceptive pill works, and you will need to take an increased dose. As with PEP, you need to take the pill within 72 hours of having sex, and ideally sooner.

An alternative and very effective method of emergency contraception is to have an intrauterine device (IUD) fitted. Talk to your doctor about the best option for you. 



Pre-exposure prophylaxis (PrEP) is a form of HIV prevention that uses medication to protect HIV-negative people from acquiring HIV. It means people take anti-HIV drugs when they are at risk of exposure to HIV.

The principle of PrEP is similar to that of antimalarial tablets, used to prevent malaria when travelling in tropical countries. If the person taking PrEP is exposed to HIV, the anti-HIV drugs in their body stop the virus from entering cells and replicating. This prevents HIV from establishing itself and stops the person acquiring HIV.

Research shows that PrEP is highly effective in preventing the sexual transmission of HIV, as long as the drugs are taken regularly, as directed. It works for men and women, heterosexual and gay. While PrEP can prevent HIV, it does not prevent other sexually transmitted infections or pregnancy.

"PrEP could be helpful if the sex you are having is not always as safe as you would like it to be."

As well as pre-exposure prophylaxis (PrEP) you may also hear about post-exposure prophylaxis (PEP). Both use anti-HIV drugs as ‘prophylaxis’, in other words as prevention.

PrEP involves taking medicine on an ongoing basis, before possible exposure to HIV, because you are aware of an ongoing risk of HIV infection. PEP involves taking medicine for 28 days, after a specific incident that may have put you at risk of HIV. 

Taking Prep

The medications usually used as PrEP are tenofovir and emtricitabine. When these drugs are combined in a single pill it is usually called Truvada. These drugs were chosen for use as PrEP partly because they have fewer side-effects than some other antiretrovirals.

For PrEP to work well, it’s important to take the pills regularly, as directed by your doctor. An occasional forgotten dose will not make PrEP ineffective. But people who only occasionally take their pills will not be protected from HIV.

It’s usually recommended that PrEP be taken every day. This maintains protective levels of the drugs in the body.

An alternative approach is to only take PrEP around the time of having sex. This may be possible for some people who usually know in advance when they will have sex. If you are only taking PrEP around the time of having sex, it is recommended that you take:

  • a double dose (two pills) between 2 and 24 hours before sex,

  • a single dose (one pill) 24 hours later, and

  • another single dose 24 hours after that.

Taking pills this way may reduce side-effects. But with fewer overall doses, it’s especially important not to miss doses.

Ideally, PrEP should be prescribed by a doctor and taken under medical supervision. 

If you are in the UK, or another country where PrEP is not yet approved by regulatory agencies, you may not be able to obtain PrEP from a doctor. In these situations, some people have chosen to import their own PrEP drugs and arrange their own monitoring. There’s more information on this in NAM’s How to get PrEP in the UK factsheet.

It’s important to check that you don’t have HIV before beginning PrEP. While taking PrEP, regular clinic appointments are needed to check for side-effects and to repeat HIV testing, as well as for advice and support.

The effectiveness of PrEP

Several studies have shown that PrEP significantly reduces the risk of HIV infection. The World Health Organization supports the use of PrEP.

In a study done with gay men in England, daily PrEP reduced HIV infections by 86%. In a study with gay men in France, PrEP taken before and after sex also reduced infections by 86%.

PrEP can work for heterosexual men and women too. In a study done with couples in Africa, it reduced infections by 75%. But some other studies had less impressive results, because too many of the people taking part did not take PrEP regularly.

In studies, when PrEP appears not to have worked for someone, this was because the person was not actually taking the medication. In people who are able to take PrEP regularly, only occasionally missing doses, PrEP appears to prevent almost 100% of infections.

The availability of PrEP

PrEP has been available in the US since 2012. At the moment, PrEP is not generally available in the UK or in other European countries with the exeption of France, which approved PrEP in late 2015.

Individuals living in the UK can purchase PrEP drugs from overseas and import them into the UK. For more information on this, read NAM’s How to get PrEP in the UK factsheet.

When PrEP is made available, there will probably be guidelines on who can receive it. It may be recommended for individuals who are considered to be at increased risk of acquiring HIV. It won’t be recommended for everyone.

Healthcare providers may need to ask people questions about their sexual behaviour and sexual partners before prescribing PrEP.

Would PrEP be right for you?

PrEP could be helpful if the sex you are having is not always as safe as you would like it to be. If it is sometimes difficult to use condoms or to be sure of your partner’s HIV status, PrEP could make the sex safer. If it’s likely that some of your sexual partners have HIV without realising it, then PrEP could help protect your health.

People are not expected to take PrEP forever. PrEP is most likely to be useful for a period of months or years when the risk of HIV is greatest. If you no longer feel at risk you can discuss stopping PrEP with your healthcare provider. You’ll be able to re-start it later.

While PrEP provides extra protection against HIV, it does not protect against other sexually transmitted infections. Condoms can provide protection against gonorrhoea, chlamydia, syphilis and hepatitis C, as well as preventing unwanted pregnancy.

PrEP isn’t the right choice for everyone. People who are able to consistently use condoms and other HIV prevention strategies don’t need PrEP.

If your partners definitely don’t have HIV, then PrEP isn’t needed. And if you have a partner who is living with HIV, taking HIV treatment and has an undetectable viral load, then the risk of HIV transmission is extremely low anyway.

If it would be difficult for you to take pills regularly (either every day or every time you have sex) without missing doses, then PrEP would not be suitable. It might not be suitable for people who dislike the idea of relying on medication or don’t want to go to regular medical appointments.

Side-effects and resistance

Around one-in-ten people taking Truvada as PrEP experience short-term side-effects in the first few weeks of taking it. These problems, such as stomach problems, headaches and tiredness, are usually minor and go away after a week or two.

Long-term side-effects are rare. Up to 2% of people taking PrEP have small decreases in kidney function while taking it. This usually returns to normal after stopping taking PrEP. Healthcare providers monitor the kidney function of people taking PrEP.

While some people have expressed concern about the use of PrEP leading to the development of drug-resistant HIV, this has rarely happened in practice.