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Using anti-HIV drugs to prevent HIV

Undetectable viral load and infectiousness

The goal of HIV treatment is an undetectable viral load. An undetectable viral load means that your blood has a level of HIV below the level which can be measured by viral load tests. This does not mean that you have been cured of HIV, but that the combination of drugs you are taking has reduced HIV's ability to reproduce so much that it can only be detected in very low levels in your blood.

HIV treatment also lowers the amount of virus in other body fluids, including semen and vaginal fluids.

Large research studies in recent years have shown that the risk of HIV transmission is greatly reduced when people are taking HIV treatment and have an undetectable viral load. (This is sometimes referred to as HIV ‘treatment as prevention’, or TasP.) Doctors now advise that, in the right circumstances, taking HIV treatment is as effective in preventing HIV as when condoms are used properly.

These circumstances are that:

  • neither you nor your partner has any sexually transmitted infections (you would both need to have sexual health check-ups)
  • you have had an undetectable viral load for more than six months and you take your HIV treatment as advised by your healthcare team
  • you have your viral load tested regularly.

If this is the case for you and your partner, being on HIV treatment may be an effective way of preventing HIV transmission.

If you and your partner want to stop using condoms, it is important to discuss the level of safety you are both comfortable with before you stop using condoms. You may need to explain or discuss what an undetectable viral load means with HIV-negative partners. You may both need some time to understand the situation clearly before you come to a decision.

It is worth remembering that HIV treatment does not reduce the risk of the transmission of other sexually transmitted infections (STIs). Condoms remain the most effective way to prevent most other STIs. 

When you first meet a new partner, it’s usually impossible to know whether the other person has an STI or not (there may be no symptoms). The only reliable way to know whether or not you or your partner has an STI is for both of you to have a comprehensive sexual health check-up. If either of you has had sex with a third person since the check-up, it needs to be repeated.

If you are not yet taking HIV treatment for your own health, but you decide you would like to start treatment to reduce the risk of passing on HIV to partners, talk to your HIV doctor. They should respect this decision and prescribe HIV treatment. If they question your need for treatment, explain that you wish to protect partners from your HIV.

You can find out more about HIV treatment as prevention on NAM’s website, including the research studies showing its effectiveness and more detailed advice to help you make any decisions.


If someone has been exposed to HIV during sex, they can take a short course of anti-HIV drugs to try to prevent infection. This is called post-exposure prophylaxis, or PEP for short. This course of drugs can be prescribed by sexual health, genitourinary medicine (GUM) and HIV clinics, or hospital accident and emergency departments out of hours, if there has been significant risk. The professional organisation of the UK's sexual health doctors (the British Association for Sexual Health and HIV – BASHH) has guidelines recommending the circumstances in which PEP should be used.

There needs to be a significant risk of HIV infection. Whether or not the risk is ‘significant’ will depend on the type of sex you have had, and other circumstances. Where the HIV-positive partner is known to be on HIV treatment with an undetectable viral load, PEP is not recommended. (See BASHH’s website for more information.)

You can assess the risk of HIV transmission using a tool on the Terrence Higgins Trust website.

It is important to get and take PEP as soon as possible after potential exposure to HIV – ideally within 24 hours, and certainly within 72 hours. PEP isn’t 100% effective. However, there have been very few reports of HIV infection after the use of PEP. PEP is considered to be an emergency treatment. It should be free of charge regardless of immigration status.

If you are taking HIV treatment and have sex without a condom 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 acquiring HIV. This is not a good idea.

Some anti-HIV drugs work better as PEP than others. A full PEP course should last a month. Sharing your drugs could be risky as some, 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 have 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.

You can read more about PEP on NAM’s website.


Pre-exposure prophylaxis (PrEP) is another way of preventing HIV infection. HIV-negative people can take some doses of anti-HIV drugs when they are at risk of exposure to HIV. Recent research has shown this is very effective in lowering the risk of infection, as long as the drugs are taken regularly and exactly as prescribed. Evidence of PrEP’s effectiveness is particularly strong for men who have sex with men.

However, PrEP does not protect people from other sexually transmitted infections or pregnancy.

The anti-HIV drug currently used as PrEP is Truvada, a pill containing two drugs, tenofovir and emtricitabine. Truvada was chosen because these two drugs have few side-effects and are less likely to cause problems with drug resistance if you have already become infected with HIV by the time you start taking PrEP, or if it fails to prevent infection.

Although there are other effective ways of preventing HIV transmission, PrEP may provide a useful option for some people, at some points in their life. For example, studies have shown that it can be a good prevention method for gay men at times when their sexual activity puts them at high risk of HIV, rather than being a long-term option.

The use of PrEP to prevent HIV transmission in different situations is still being studied in the UK. PrEP has been shown to be highly effective at preventing HIV in people at high risk (such as men who have sex without a condom with other men). The availability of PrEP on the NHS is currently being debated by the Government. Some clinics are offering PrEP with a private prescription. Talk to staff at a sexual health clinic if you think PrEP would be a good option for you. 

You can follow news on research studies on PrEP and its use and availability in the UK on NAM's website.

HIV & sex

Published January 2016

Last reviewed January 2016

Next review January 2019

Contact NAM to find out more about the scientific research and information used to produce this booklet.

This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.
Community Consensus Statement on Access to HIV Treatment and its Use for Prevention

Together, we can make it happen

We can end HIV soon if people have equal access to HIV drugs as treatment and as PrEP, and have free choice over whether to take them.

Launched today, the Community Consensus Statement is a basic set of principles aimed at making sure that happens.

The Community Consensus Statement is a joint initiative of AVAC, EATG, MSMGF, GNP+, HIV i-Base, the International HIV/AIDS Alliance, ITPC and NAM/aidsmap

This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.

NAM’s information is intended to support, rather than replace, consultation with a healthcare professional. Talk to your doctor or another member of your healthcare team for advice tailored to your situation.