How does PrEP (for HIV) work?

4th July, 2019 • 8 min read

What is PrEP?

PrEP (or pre-exposure prophylaxis) is a medication taken by people who are not infected with the HIV virus before sex to reduce the risk of getting HIV. The treatment is still being trialled in England, but is widely available in Scotland, Wales, the United States, France, and Germany.

Studies have shown that PrEP medication can be up to 90% effective at preventing HIV infection when taken correctly, and the treatment is currently recommended for anyone who is HIV negative but at high risk of catching the HIV virus.

PrEP is normally provided as a pill. See your doctor if you would like to consider taking PrEP.

In this article, you’ll learn how PrEP works and how to access PrEP medication in your country. You’ll also learn how to make sure PrEP treatments are effective, and who should be taking PrEP every day.

How does PrEP work?

PrEP is a preventative treatment, which means that it does not cure HIV or reverse pre-existing infections. Instead, PrEP medications reduce the risk of ‘catching’ HIV by blocking the virus before it can spread in your body.

PrEP pills normally contain a combination of two antiretroviral drugs like tenofovir and emtricitabine. These drugs inhibit the production of an enzyme called reverse transcriptase, which the HIV virus needs to replicate itself and start colonising your cells.

Antiretroviral drugs like tenofovir and emtricitabine build up in your blood and tissues over time, which is why PrEP is normally taken for some time before you engage in activities that may expose you to the HIV virus.

If you do not take enough of your PrEP medication or you miss several doses of a prescribed treatment, there may not be enough medicine in your bloodstream to block the HIV virus.

How effective is PrEP?

If PrEP medications are taken correctly, studies show that they can be up to 90% effective at preventing HIV infections.

However, studies conducted in Kenya, Tanzania, and Botswana have also shown that taking PrEP medications incorrectly makes them much less effective.

If you are taking PrEP medications, it is important to take them regularly. Missing doses may increase your chances of catching HIV. This is particularly important if you are taking PrEP on-demand, as you will take fewer total doses and missing a pill means significantly reducing the amount of antiviral medications in your system.

Note: PrEP medications will not protect you from other sexually transmitted infections. To reduce your chances of contracting a disease like

, chlamydia, or
, you should still practice safe sex.

Who should take PrEP?

According to guidance issued by the World Health Organisation (WHO), the British Association for Sexual Health and HIV (BASHH), and the Centers for Disease Control and Prevention (CDC), PrEP should be taken by anyone who is at high risk of contracting HIV. This includes anyone who:

  • is in an ongoing sexual relationship with someone who is HIV positive
  • isn’t in a mutually monogamous relationship with someone who recently tested HIV-negative and is a gay or bisexual man who has unprotected anal sex or has been diagnosed with a sexually transmitted illness over the past six months
  • isn’t in a mutually monogamous relationship with someone who recently tested HIV-negative and is a heterosexual man or woman who has unprotected sex with partners whose HIV status is not known, where the partners are also at an increased risk from HIV

This also includes men who have sex with men, trans women, trans men, or hetrosexual couples where one partner is known to be HIV positive.

If you are struggling with a drug addiction and you are worried about contracting HIV, please see your doctor for advice.

Is there anything I need to do before I take PrEP?

Before you can start taking PrEP, you need to be screened for:

  • kidney function, to check that your kidneys are functioning properly before you start taking PrEP
  • the presence of HIV, because a pre-existing HIV infection should be treated with antiretroviral therapy
  • other sexually transmitted infections
  • hepatitis B
    (HBV) infections

Most PrEP treatments also help to suppress the hepatitis B virus but they need to be taken more carefully, and you will need to be supervised by a medical professional if you already have HBV.

Your doctor will be able to order these tests for you. Your local sexual health or GUM clinic may also be able to help with the STI and HIV tests, or you could use an at-home testing service.

How can I get PrEP, and is it free?

Access to PrEP medication depends on the country you live in. If you are unsure how to access PrEP, you should contact your doctor or a local sexual health clinic as soon as possible. They will be able to explain your treatment options and help you get started with a course of PrEP medication, if appropriate.

Below, you will find some guidance that’s designed to help you access PrEP in a select handful of countries.

The UK

If you live in Scotland, Wales or Ireland you may be able to access PrEP treatments at your local sexual health clinic. If you meet the eligibility criteria and agree to attend regular screening sessions you should be able to access PrEP treatments for free.

Find sexual health clinics near you

PrEP is still being trialled in England, but you may be eligible to receive PrEP as part of the

NHS PrEP Impact Trial

You can pay to import PrEP medications from online pharmacies, but you should take care when doing so. Always use a trusted website and make sure that you consult a medical professional before you start taking PrEP. PrEP medications are only effective if they are taken correctly, and it’s important to make sure you are taking the right kind of PrEP pills before you start your course. You may also need blood tests before starting on PrEP, so see your doctor for advice first.

Online pharmacies are often unregistered, and may not abide by the stringent safety standards that regular pharmacies in the UK have to comply with. Many NHS sexual health clinics will provide support and assistance for people buying PrEP online, so contact your local clinic if you are thinking of buying PrEP from an online pharmacy.

You should also check your HIV status before you start taking a PrEP medication. You can do this by visiting your local doctor.

The United States

PrEP treatment is widely available in the United States. If you fit the eligibility criteria and want to learn more about accessing PrEP, you can use a service like

PrEP Locator
to find a local provider.

If you

need help paying for PrEP
, the CDC offer help and guidance.


Generic and brand name PrEP medications are available from pharmacies throughout Germany. You will need a valid prescription from your doctor first. This means you will need to book an appointment with your doctor so they can make sure that PrEP is a good fit for you.


PrEP is widely available in France, and the full cost of your medication will be covered by the French healthcare system. However, you do need a prescription to obtain PrEP in France, so if you are worried about HIV or think you are at risk of contracting the virus, book an appointment with your doctor straight away.


Generic PrEP medications have been approved for use in India, but PrEP is not widely available and many people struggle to access the treatment. According to PrEP Watch, only 600-800 people are currently using PrEP in India.

If you live in India and want to learn more about accessing PrEP, you can contact the

The Humsafar Trust
or the
India HIV/AIDS Alliance
. These advocacy organisations are currently working to improve access to PrEP medications in India and may be able to:

  • point you towards PrEP resources
  • let you know when PrEP becomes locally available
  • provide you with additional guidance or advice


PrEP is approved for use in Australia, and PrEP medications can now be accessed from pharmacies all over the country. PrEP medications are also subsidised under the Pharmaceutical Benefits Scheme (PBS) so people with a medicare card will be able to access the treatment at a reduced cost.

If you live in Australia and think you might be at risk of contracting HIV, you should book an appointment with your doctor and ask them about getting started with PrEP.

Other countries

If you live in a country that isn’t listed here, you may be able to access up-to-date information about the availability of PrEP using a resource like

PrEP Watch
, which tracks global PrEP use and access statistics.

is another useful resource that provides detailed information about PrEP access for a number of Pacific and Asian countries.

Important: Our website provides useful information but is not a substitute for medical advice. You should always seek the advice of your doctor when making decisions about your health.