HIV PREVENTION

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Pre-exposure prophylaxis (PrEP) is medication used to prevent HIV. It contains antiretroviral (ARV) drugs so that if you have an exposure to HIV, the virus can’t reproduce or spread throughout the body, and it will eventually die.[1]

PrEP should only be used by people who are not living with HIV, and when PrEP is taken as prescribed it is highly effective, reducing the likelihood of HIV transmission by about 99%.[1–3] Therefore, the chance of acquiring HIV from someone on PrEP, who is taking it as prescribed and is not an iv drug user, is extremely low.

Who should take PrEP?

PrEP can be used by anyone who is not living with HIV, but may have a higher likelihood of acquiring HIV. This includes:[2]

  • People in sexual relationships with an HIV-positive partner.
  • People who have condomless sex with a partner(s) and do not know their HIV status.
  • People who exchange sex for money, drugs, shelter, or another reason.
  • People who inject drugs.

If you have a partner who is on antiretroviral therapy (ART) and has an undetectable viral load, then you cannot contract HIV from them, therefore, you may not need PrEP.[4] This concept, known as Undetectable = Untransmittable (U=U), means that when HIV levels are low enough to be undetected by a test, the virus cannot be transmitted sexually.[4,5]

If you believe you are likely to contract HIV, speak to your healthcare provider about beginning PrEP. When deciding if PrEP is right for you, you and your healthcare team will discuss topics such as sexual history, drug use, health history, potential side effects, and any other concerns.[3] It is always important to be honest with your healthcare professional to ensure you have the most effective and appropriate HIV prevention strategy.

If you are trying to get pregnant or are already pregnant, talk to your doctor about the risks and benefits of PrEP. Do not stop taking it without speaking to your doctor.[6]

If you are living with HIV, then you shouldn’t take PrEP. Taking PrEP while HIV positive could lead to drug resistance, which may limit future HIV treatment options. This means the HIV virus could become more difficult to treat.[3,7,8]

How Is PrEP taken?

PrEP is primarily available in two forms:[3]

  • Oral pills

  • Long-acting injections

Taking oral PrEP daily provides protection for both vaginal/frontal and anal sex.[9] If having vaginal/frontal or anal sex, you should take 1 tablet every day at around the same time, and ensure that you take 7 consecutive doses before you have sex.[9]

If you think you may have sex before the first 7 days, then you can take 2 tablets at least 2 hours before the encounter.[9] However, this approach does not apply to vaginal/frontal sex, which requires at least 7 days of daily PrEP to be effective.[10]

You can also take on-demand PrEP instead of taking PrEP daily, although this is only suitable for anal sex, not vaginal/frontal sex.[10,11] This is because on-demand PrEP has only been studied and proven effective for anal sex in men who have sex with men.[10,11]

On-demand PrEP is also sometimes known as the “2-1-1” schedule.[10]

PrEP injections are needed less frequently than oral PrEP.[12]

Inconsistent PrEP usage

Missed PrEP doses can result in insufficient medication levels in the bloodstream. This reduces its effectiveness at preventing HIV, thereby increasing the likelihood of HIV acquisition.[2]

If you forget to take your daily oral doses often, setting a daily alarm to remind you may be helpful.[9]

How long can you take PrEP for?

You can take PrEP for as long as you want or need, but some people might choose to stop if their circumstances change.[13] There is varying guidance regarding when to safely stop PrEP after the last time unprotected sex occurred.[13] So, you should talk to your doctor about how to stop PrEP safely and discuss other effective HIV prevention strategies moving forward.[13]

Does PREP prevent sexually transmItted InfectIons (STI)s?

PrEP is effective at preventing HIV acquisition, but several studies show that PrEP does not prevent other STIs.[3]

PrEP users still need to follow safe sex practices, such as consistently using condoms, which remain the most effective strategy to prevent STIs.[3,14] Schedule regular sexual health check-ups to test for STIs.[9]

Other factors to consIder when taking PrEP

PrEP is very effective at preventing HIV transmission, and some studies report that with perfect adherence it can reduce the chances of acquiring HIV by 99%, the chances of acquiring HIV are higher for iv drug users.[3] While no prevention method is 100% effective, including condoms, PrEP provides a very high level of protection when used correctly.[3,15,16]

To ensure comprehensive protection, additional preventive measures are recommended. These include using condoms, practising safe sex—such as avoiding alcohol or drug use during sexual activity (chemsex)—and maintaining open, honest communication between partners.[17,18]

There are certain situations where HIV transmission could still occur from an individual using PrEP, these include:

Being HIV positive

An HIV test is needed before starting PrEP to ensure individuals are HIV-negative.[3,9] However, some individuals on PrEP may be unknowingly living with HIV, as they may have been in the 'window period' and tested negative when PrEP was prescribed.[19]

The window period is the time between when a person first acquires HIV and when the virus can be detected by a test. The window period ranges from 45 days to 90 days depending on the type of HIV test.[19]

Although rare, individuals can also contract HIV within the time they test negative and when they start PrEP.[3,8,20] This is why PrEP must be started a maximum of seven days after an HIV-negative result.[3]

In such cases, HIV transmission remains possible because PrEP is designed to prevent new infections but is not strong enough to suppress the viral load in someone already living with HIV.[8,20]

People using PrEP should undergo regular HIV testing to confirm they are still HIV-negative.[3,5]

PrEP side effects

As with all medications, there can be some side effects of taking PrEP. These include the common side effects of nausea, diarrhoea, bloating, indigestion, headache, feeling tired or weak, and sleeping problems.[1,2] However, if they do occur, they’re usually mild and resolve within the first one to three months.[1,2]

Other ways to prevent HIV

There are also ways to prevent HIV transmission that do not include the use of drugs, these include:[21]

Always using condoms during sex.

Both partners getting tested for HIV before sex.

Never sharing needles or other drug injection equipment.

Abbreviations:

ART, antiretroviral therapy; PrEP, pre-exposure prophylaxis; STI, sexually transmitted infection.

References:

  1. San Francisco AIDS Foundation. PrEP Facts: What is PrEP? 2021. Available from: https://www.sfaf.org/collections/beta/prep-facts-what-is-prep/ [Accessed: March 2025]
  2. NHS. About Pre-Exposure Prophylaxis (PrEP). Available from: https://www.nhs.uk/medicines/pre-exposure-prophylaxis-prep/about-pre-exposure-prophylaxis-prep/ [Accessed: March 2025]
  3. NIH. Sundareshan V, Swinkels HM, Nguyen AD, Mangat R, Koirala J. Preexposure prophylaxis for HIV prevention. 2024. Available from: https://www.ncbi.nlm.nih.gov/books/NBK507789/ [Accessed: March 2025]
  4. HIV.gov. Viral Suppression and an Undetectable Viral Load. 2025. Available from: https://www.hiv.gov/hiv-basics/staying-in-hiv-care/hiv-treatment/viral-suppression [Accessed: March 2025]
  5. NHS. Who can and cannot take Pre-Exposure Prophylaxis (PrEP). 2023. Available from: https://www.nhs.uk/medicines/pre-exposure-prophylaxis-prep/who-can-and-cannot-take-pre-exposure-prophylaxis-prep/ [Accessed: March 2025]
  6. NHS. Pregnancy, breastfeeding and fertility while taking Pre-Exposure Prophylaxis (PrEP). 2023. Available from: https://www.nhs.uk/medicines/pre-exposure-prophylaxis-prep/pregnancy-breastfeeding-and-fertility-while-taking-pre-exposure-prophylaxis-prep/ [Accessed: March 2025]
  7. CROI Conference. IMPACT OF PrEP ON DRUG RESISTANCE AND ACUTE HIV INFECTION, NEW YORK CITY, 2015-2017. 2019. Available from: https://www.croiconference.org/abstract/impact-prep-drug-resistance-and-acute-hiv-infection-new-york-city-2015-2017/ [Accessed: March 2025]
  8. AIDSMAP. Cairns G. PrEP and drug resistance. 2020. Available from: https://www.aidsmap.com/about-hiv/prep-and-drug-resistance [Accessed: March 2025]
  9. NHS. How and when to take Pre-Exposure Prophylaxis (PrEP). 2023. Available from: https://www.nhs.uk/medicines/pre-exposure-prophylaxis-prep/how-and-when-to-take-pre-exposure-prophylaxis-prep/ [Accessed: March 2025]
  10. CDC. Preventing HIV with PrEP. 2024. Available from: https://www.cdc.gov/hiv/prevention/prep.html [Accessed: March 2025]
  11. AIDSMAP. Highleyman L. On-demand PrEP is highly effective, but some may find it confusing. 2020. Available from: https://www.aidsmap.com/news/jul-2020/demand-prep-highly-effective-some-may-find-it-confusing [Accessed: March 2025]
  12. AIDSMAP. Alcorn K, Pebody R. Cabotegravir injections for PrEP. 2024. Available from: https://www.aidsmap.com/about-hiv/cabotegravir-injections-prep [Accessed: March 2025]
  13. Rutstein SE, Smith DK, Dalal S, Baggaley RC, Cohen MS. Initiation, discontinuation, and restarting HIV pre-exposure prophylaxis: ongoing implementation strategies. Lancet HIV. 2020 Oct 1;7(10):e721–30. doi: 10.1016/S2352-3018(20)30203-4.
  14. WHO. Sexually transmitted infections (STIs). 2024. Available from: https://www.who.int/news-room/fact-sheets/detail/sexually-transmitted-infections-(stis) [Accessed: March 2025]
  15. Cambou MC, Landovitz RJ. Challenges and opportunities for preexposure prophylaxis. Top Antivir Med. 2021 Oct 1;29(4):399–406. PMID: 34856093
  16. NHS. Condoms. 2024. Available from: https://www.nhs.uk/contraception/methods-of-contraception/condoms/ [Accessed: March 2025]
  17. NIH. HIV and Sexually Transmitted Infections (STIs). 2021. Available from: https://hivinfo.nih.gov/understanding-hiv/fact-sheets/hiv-and-sexually-transmitted-diseases-stds [Accessed: March 2025]
  18. Planned Parenthood. How Do You Prevent HIV / AIDS? 2025. Available from: https://www.plannedparenthood.org/learn/stds-hiv-safer-sex/hiv-aids/how-can-i-prevent-hiv [Accessed: March 2025]
  19. AIDSMAP. Pebody R. What is the window period for HIV testing? 2021. Available from: https://www.aidsmap.com/about-hiv/what-window-period-hiv-testing [Accessed: March 2025]
  20. AIDSMAP. Cairns G. PrEP may make it hard to tell if someone has an HIV infection. 2018. Available from: https://www.aidsmap.com/news/may-2018/prep-may-make-it-hard-tell-if-someone-has-hiv-infection [Accessed: March 2025]
  21. NIH. HIV Prevention: The Basics of HIV Prevention. 2023. Available from: https://hivinfo.nih.gov/understanding-hiv/fact-sheets/basics-hiv-prevention [Accessed: March 2025]

NP-GBL-HVX-COCO-250012 September 2025

Reporting of side effects

If you get any side effects, talk to your doctor, pharmacist, or nurse. This includes any possible side effects not listed in the package leaflet. You can also report side effects directly via the GSK Reporting Tool link https://gsk.public.reportum.com/. By reporting side effects, you can help provide more information on the safety of this medicine.

If you are from outside the UK, you can report adverse events to GSK/ ViiV by selecting your region and market, here.