Pre-exposure prophylaxis (PrEP)

Image by Doctor 4U. Creative Commons licence.

Key points

  • PrEP is highly effective in preventing the sexual transmission of HIV.
  • For PrEP to work well, it’s important to take the pills as directed.
  • While PrEP can prevent HIV, it does not protect against other sexually transmitted infections.
  • As well as PrEP tablets, PrEP vaginal rings will be available soon.
  • Different PrEP delivery methods such as injectables and implants are being researched.

Pre-exposure prophylaxis (PrEP) is a form of HIV prevention that uses anti-HIV drugs to protect HIV-negative people from acquiring 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 from acquiring HIV.

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

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 use of medicine before and after exposure to HIV, because you are aware of the risk of being exposed to HIV. PEP involves taking medicine after possible exposure to HIV for 28 days. There’s more information about PEP in NAM’s Post-exposure prophylaxis factsheet.

The US Food and Drug Administration first approved the use of the emtricitabine/tenofovir disoproxil fumarate combination pill (Truvada) as PrEP in 2012. It has been approved by many other regulatory agencies since then. Currently, it is the most studied and widely accessible PrEP option. Nonetheless, there are different combinations of anti-HIV drugs and other methods that are available or under research to be used as PrEP.

Taking PrEP

Ideally, PrEP should be prescribed by a doctor and taken under medical supervision. This is because there are important things to consider before starting PrEP. You need to take an HIV test to check your HIV status. This is to ensure you don’t have HIV before you start taking PrEP.

Tests for sexually transmitted infections, kidney function, hepatitis B and hepatitis C are also recommended. Healthcare providers may need to ask you additional questions about your sexual behaviour and sexual partners before prescribing PrEP.

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 for people who cannot adhere to their treatment and miss pills regularly, PrEP may not work.

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.

PrEP options

Currently PrEP tablets and PrEP vaginal rings are approved for use. Which PrEP option is available and more suitable for you depends on where you live, who you are and the way you want to use PrEP.

There are two combinations of anti-HIV drugs approved to be used as PrEP tablets: emtricitabine and tenofovir disoproxil fumarate (also known as Truvada), and emtricitabine and tenofovir alafenamide (also known as Descovy).

Tablets: emtricitabine/tenofovir disoproxil fumarate

It’s usually recommended that emtricitabine/tenofovir disoproxil fumarate (Truvada) be taken every day as PrEP. This maintains protective levels of the drugs in the body. When taken daily, it works for men and women, cisgender and transgender, heterosexual and gay.

You can take take daily PrEP for a short block of time when you know that you might be at increased risk of HIV - like when going on holiday.

Susan Cole, Matthew Hodson and Mercy Shibemba talk about the different forms of PrEP available.

For gay and bisexual men (who were assigned male at birth) there is also an alternative approach for using emtricitabine/tenofovir disoproxil fumarate as PrEP, called event-based dosing or on-demand PrEP. This is to take PrEP around the time of having sex. This may be more suitable for men who usually know in advance when they will have sex and when they might come into contact with HIV. If you are following event-based dosing, 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.

If you have sex on several days in a row, you need to continue taking a single dose every 24 hours until two days after your last sex.

Event-based dosing is not recommended for those at risk of acquiring HIV through vaginal sex. This is because there isn’t enough data on this approach maintaining protective levels of drugs in vaginal tissues.

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

Tablets: emtricitabine/tenofovir alafenamide

The US Food and Drug Administration approved the use of emtricitabine/tenofovir alafenamide (Descovy) in male adults and adolescents as daily PrEP in 2019. Currently, Descovy is not recommended for use in people who were assigned female at birth. This is because the effectiveness has not been studied for people who are at risk of acquiring HIV through vaginal sex.

These tablets are not yet available as PrEP in most countries. Due to the different formulation of the tenofovir, you may experience different side effects compared to emtricitabine/tenofovir disoproxil fumarate. In a study, emtricitabine/tenofovir alafenamide was found less likely to cause changes to kidney and bone health. The same study also showed that people using emtricitabine/tenofovir alafenamide put on more weight and had poorer cholesterol outcomes. You can find more information on use of Descovy as PrEP here.

Vaginal rings

In July 2020, European Medicine Agency (EMA) gave a positive opinion on the use of the vaginal ring containing the anti-HIV drug dapivirine as PrEP. In January 2021, the World Health Organization recommended it as an additional HIV prevention option.

It is a ring made of silicone containing the anti-HIV drug dapivirine. Similar to a contraceptive ring, it is inserted in the vagina and slowly releases dapivirine over the course of a month. After a month you will need to replace it.

The vaginal rings are not yet available. With the approval from the EMA they are expected to become available soon in low- and middle-income countries. You can find details on the use of vaginal rings as PrEP here.

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 (emtricitabine/tenofovir disoproxil fumarate and the dapivirine vaginal ring).

PrEP can work for heterosexual men and women. 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.


pre-exposure prophylaxis (PrEP)

Antiretroviral drugs used by a person who does not have HIV to be taken before possible exposure to HIV in order to reduce the risk of acquiring HIV infection. PrEP may either be taken daily or according to an ‘event based’ or ‘on demand’ regimen. 


How well something works (in real life conditions). See also 'efficacy'.


An umbrella term for people whose gender identity and/or gender expression differs from the sex they were assigned at birth.


Taking a drug to prevent an illness. Primary prophylaxis is the use of drugs to prevent a first occurrence of illness. Secondary prophylaxis is the use of drugs to prevent re-occurrence of illness. Pre-exposure prophylaxis (PrEP) and post-exposure prophylaxis (PEP) for HIV are both forms of primary prophylaxis.

post-exposure prophylaxis (PEP)

A month-long course of antiretroviral medicines taken after exposure or possible exposure to HIV, to reduce the risk of acquiring HIV.

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.

In a study done with gay men in England, use of Truvada as 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%.

In a study, daily Truvada was found less effective for trans women. This was due to lower adherence and concern among trans people about interactions between daily Truvada and hormone therapy. However, interactions are unlikely due to differences in metabolism and clearance between PrEP and feminising hormones. Currently, there are no data on effectiveness of daily Truvada for trans men.

The effectiveness of daily Descovy has only been assessed in one study, which found it to have similar results to daily Truvada, in men who have sex with men.

The availability of PrEP

Currently PrEP is available in many countries in Europe, east Africa, North America, east Asia and Australasia. It is also available in some countries in Latin America, south-east Asia, and west Africa.

How you can access PrEP and how much you will need to pay for the treatment depends on where you live and the health system in your country. For example, you can access PrEP through private insurance in the US and through the NHS in the UK.  You can find the full list of countries with PrEP programmes here.

You can ask your healthcare provider if there are any PrEP programmes or trials through which you can access PrEP without any charge.

PrEP research

In addition to PrEP tablets and vaginal rings, there are also other delivery methods under research such as injectables and implants.

In a study, an injection of the anti-HIV drug cabotegravir every eight weeks was found more effective than PrEP tablets in gay and bisexual men and trans women. Another study is currently being conducted to test the efficacy of injectable PrEP for women, who were assigned female at birth, in sub-Saharan Africa. You can find more details here.

Implants containing the new anti-HIV drug islatravir may offer a new option for those who have difficulties taking tablets. A small implant containing islatravir which would be inserted under the skin on the upper arm is under research. It may prevent HIV infection for over a year. You can find more details of the study here.

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, there is no risk of HIV transmission 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.

Gender-affirming hormone therapy

Many transgender people considering using PrEP have concerns about side effects and drug interactions, and in particular whether PrEP drugs might interfere with gender-affirming hormone therapy. Studies show that PrEP tablets do not raise or lower levels of gender-affirming hormones. Hormones taken by transgender women appear to slightly lower levels of the PrEP drug tenofovir, but not enough to affect the efficacy of daily PrEP. There’s more information on another page.


Full image credit: PrEP Emtricitabine/Tenofovir (Generic Truvada) PrEP Treatment Image 11. Image by Doctor 4U. Available at under a Creative Commons licence CC BY 2.0.

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