PrEP — how HIV prevention works
Reviewed by the My London Pharmacy clinical team, Yousef Yaghoubi, Pharmacist
·29 July 2026
·7 min read
Quick answer
PrEP, or pre-exposure prophylaxis, is a medicine taken by HIV-negative people to greatly reduce the risk of acquiring HIV. Taken as prescribed, it is highly effective. It involves regular testing and monitoring, may be taken daily or around specific events, and works best alongside other prevention such as condoms and regular STI checks.
What PrEP is
PrEP stands for pre-exposure prophylaxis. It is a medicine taken by people who do not have HIV to protect themselves from acquiring it. The word prophylaxis simply means prevention, and pre-exposure means it is taken before any possible contact with the virus. The tablets contain antiretroviral medicines that stop HIV from establishing an infection in the body if you are exposed to it.
PrEP has become an important part of HIV prevention in the UK and is available through sexual health services. It is designed for people who are HIV-negative and at higher chance of exposure, and it is one of several tools, alongside condoms, regular testing and treatment for people living with HIV, that together have helped reduce new HIV infections.
It is worth being clear about what PrEP is not. It is not a vaccine, so it only works while you are taking it as prescribed, and it is not a treatment for people who already have HIV, who need a different combination of medicines. Understanding this helps explain why testing before and during PrEP is such an important part of using it safely.
How PrEP works
When taken correctly, PrEP keeps a protective level of medicine in the body's tissues. If you are exposed to HIV, these medicines block the virus from copying itself and taking hold, so an infection does not develop. This is different from treatment for HIV, which is taken by people who already have the virus.
The protection PrEP offers is very high when it is taken as prescribed, which is why following the dosing advice matters so much; missed doses lower the level of protection. It is also important to understand that PrEP protects only against HIV, not against other sexually transmitted infections or unplanned pregnancy, which is why it works best as part of a wider approach to sexual health.
Who might consider PrEP
PrEP may be suitable for HIV-negative people who have a higher chance of exposure to HIV. This can include men who have sex with men who have condomless sex, people whose partner is living with HIV and does not have a fully suppressed viral load, and others who feel they may be at increased risk.
Deciding whether PrEP is right for you is a personal choice made with a clinician, who can talk through your circumstances confidentially and without judgement. Everyone's situation is different, and a clinician can advise whether PrEP may be suitable for you and how it fits with your other prevention choices. There is no need to justify or over-explain; the conversation is simply about matching prevention to your life.
How PrEP is taken
There are two main ways to take PrEP. Daily dosing means taking one tablet every day, which suits people who have sex regularly and provides steady, ongoing protection. Event-based dosing, sometimes used by men who have sex with men, involves taking tablets around the time of sex following a specific schedule advised by a clinician.
The right approach depends on your circumstances, the type of sex you have, and your preferences. A clinician will explain exactly how to take PrEP so that it reaches and maintains the level of protection needed, and how to start and stop it safely. Taking it as prescribed is what makes it effective, so it helps to build it into a routine, for example linking a daily dose to an everyday habit so it is easier to remember.
Testing and monitoring
PrEP is provided alongside regular check-ups, which are an essential part of using it safely. Before starting, you will have an HIV test to confirm you are HIV-negative, because PrEP alone is not the right treatment for someone who already has HIV. Tests of kidney function and for other STIs and hepatitis are also usually done.
While taking PrEP, you attend for regular reviews, typically every few months, to repeat HIV testing, check your kidneys are coping well, and screen for other STIs. These appointments keep PrEP working safely and are a good opportunity to discuss any questions with your clinician, including whether your current dosing approach still suits your life.
Safety and side effects
PrEP has been used by many people and is generally well tolerated. Some people notice mild side effects when they first start, such as headache, nausea or stomach upset, which usually settle within the first few weeks. Because the medicines can affect the kidneys in a small number of people, kidney function is checked before starting and at regular reviews, which is part of why ongoing monitoring matters.
If you have questions or notice anything that concerns you while taking PrEP, your clinician is the right person to speak to, as they can weigh it up against your wider health and advise whether any change is needed. For most people the balance is firmly in favour of the protection PrEP provides, but it remains an individual decision made with clinical support.
PrEP alongside other prevention
PrEP is highly effective against HIV, but because it does not protect against other infections such as chlamydia, gonorrhoea or syphilis, it works best as one part of a broader strategy. Condoms continue to offer valuable protection against a range of STIs as well as HIV, and regular STI testing helps pick up and treat other infections early.
Some people using PrEP also discuss additional measures with their clinician, such as vaccination against hepatitis where relevant. Combining PrEP with condoms, regular testing and open conversations with partners gives the most complete protection for your sexual health, and the regular reviews that come with PrEP make it easier to keep on top of the wider picture rather than thinking about HIV alone.
Stopping, restarting and changing your routine
People's lives and relationships change, and PrEP can be started and stopped as your circumstances do, provided this is done in the right way. There are recommended ways to begin PrEP so that protection builds up before you need it, and to stop it so that you remain covered for any recent exposure, and a clinician will explain the timing that applies to you and the type of sex you have.
If you are switching between daily and event-based dosing, or pausing PrEP for a while, it is worth a brief conversation with your clinician so that you stay protected throughout. Stopping PrEP does not close the door on it; many people use it during particular periods of their life and return to it later if their situation changes.
Getting started safely
If you are thinking about PrEP, the first step is a discreet, non-judgemental conversation with a clinician. They can explain how PrEP works, assess whether it may be suitable for you, arrange the tests needed before starting, and set up the ongoing monitoring that keeps it safe and effective. There is no need to feel awkward about raising it; clinicians discuss PrEP routinely and are there to help you make an informed choice.
PrEP is a positive, proactive choice for many people, and seeking advice about it is a normal part of looking after your sexual health rather than a sign that anything is wrong. It puts you in control of your own protection and sits comfortably alongside condoms and regular testing. A clinician can guide you through starting, taking and reviewing PrEP so that you feel confident and well supported throughout, including how to stop safely if your circumstances change.
Frequently asked questions
How effective is PrEP at preventing HIV?
Taken as prescribed, PrEP is highly effective at preventing HIV. Its protection depends on taking it correctly, which is why following your clinician's dosing advice matters.
Does PrEP protect against other STIs?
No. PrEP only protects against HIV. It does not prevent other infections such as chlamydia or gonorrhoea, so condoms and regular testing remain important.
Can I take PrEP only around the time of sex?
For some people, event-based dosing is an option following a specific schedule. A clinician will advise whether daily or event-based dosing suits your circumstances.
Does PrEP have side effects?
PrEP is generally well tolerated. Some people notice mild, short-lived effects such as headache or nausea when they start. Kidney function is monitored as a precaution.
What monitoring does PrEP involve?
You have an HIV test before starting and regular reviews every few months to repeat HIV testing, check kidney function, and screen for other STIs.
How do I find out if PrEP is right for me?
A clinician can discuss your circumstances confidentially, assess whether PrEP may be suitable, and arrange the tests needed before you start.