Pre-Exposure Prophylaxis (PrEP): How It Protects Against HIV
When it comes to preventing HIV, timing is everything. But unlike Post-Exposure Prophylaxis (PEP), which is taken after a possible exposure, there’s a way to protect yourself before you’re even at risk. That’s where Pre-Exposure Prophylaxis (PrEP) comes in.
PrEP flips the script on HIV prevention. Instead of waiting for a scare to act, you take it in advance, either daily or around the times you expect to be at risk. It works by building up a level of medication in your body that blocks HIV from taking hold if it enters your system. For people who may be exposed to HIV through sex or injecting drug use, it can be a game-changer.
But PrEP isn’t just a pill you pop now and then. It works best when taken consistently and with a clear understanding of how it fits into your lifestyle. It’s also not a one-size-fits-all solution, some people take it every day, while others use event-based dosing before and after sexual activity.
In this guide, we’ll break down how PrEP works in the body, who it’s recommended for, how to access it in the UK, possible side effects, and the importance of ongoing sexual health checks. We’ll also discuss why condoms still matter, even when you’re on PrEP.
What is Pre-Exposure Prophylaxis (PrEP)?
The Basics of PrEP
PrEP is a form of HIV prevention where HIV-negative people take antiretroviral medicine before potential exposure. The most common PrEP medication is a combination of two drugs - tenofovir and emtricitabine taken as a single tablet. When used correctly, PrEP is highly effective at preventing HIV transmission through sex and injecting drug use.
How It’s Taken
There are two main ways to take PrEP:
- Daily dosing: One pill every day, maintaining consistent drug levels in the body
- Event-based dosing: Sometimes called the "2-1-1" method, where doses are taken before and after sexual activity. This method is currently recommended only for men who have sex with men in certain guidelines, as research on its effectiveness in other groups is still ongoing
Effectiveness
When taken daily, PrEP can reduce the risk of getting HIV from sex by about 99%, and from injecting drug use by at least 74%. Effectiveness depends heavily on adherence, missing doses can lower protection.
How PrEP Works in the Body
The Science Behind It
HIV needs to infect certain immune cells, integrate its genetic material, and replicate to cause a lasting infection. The drugs in PrEP block a key enzyme, reverse transcriptase that the virus uses to copy its genetic material. If HIV can’t make this copy, it can’t establish infection.
Building a Protective Barrier
For daily PrEP, the medication builds up in blood and genital or rectal tissues over several days of use, creating a protective barrier. For event-based PrEP, higher loading doses before exposure aim to reach protective levels in time for risk events.
Why Consistency Matters
Drug levels need to be high enough at the time of exposure to block the virus. Missing pills can leave gaps in protection, especially for daily PrEP. Consistency is easier for some than others, people with regular routines often find daily dosing simple, while those with irregular schedules may need to set phone reminders or tie dosing to another daily habit.
Who Should Consider PrEP?
Higher-Risk Groups
PrEP isn’t intended for everyone, it’s targeted at people with a higher risk of HIV exposure. This can include:
- People with HIV-positive partners who are not on effective treatment
- People with multiple sexual partners and inconsistent condom use
- Men who have sex with men, particularly if engaging in condomless anal sex
- People who share injection equipment
- Sex works in high-prevalence areas
How Risk is Assessed
Healthcare providers consider factors like recent sexual history, frequency of condom use, partner’s HIV status, and history of STIs. Even if you don’t fit into a traditional “high risk” category, you can still discuss PrEP with a sexual health clinic if you’re concerned.
Accessing PrEP in the UK
PrEP is available free through the NHS in England, Scotland, Wales, and Northern Ireland via sexual health clinics. Some people also choose to buy it privately online, but it’s important to do this through reputable suppliers and under medical supervision.
Side Effects and Considerations
Common Side Effects
Most people tolerate PrEP well, but some experience mild side effects such as nausea, headaches, diarrhoea, or fatigue, particularly in the first few weeks. These usually settle as the body adjusts.
Long-Term Safety
Studies have shown that PrEP is generally safe for long-term use. In a small number of cases, it can affect kidney function or bone mineral density, which is why regular monitoring is recommended.
Monitoring and Follow-Up
Anyone taking PrEP should have an HIV test every three months, along with STI checks and kidney function tests. This is both to ensure PrEP is still needed and to pick up any infections or side effects early.
Follow-up and Monitoring
HIV Testing Schedule
Before starting PrEP, you must have a confirmed negative HIV test, taking PrEP while unknowingly HIV-positive can lead to drug resistance. After that, testing is recommended every three months.
STI Screening
PrEP only protects against HIV, so regular checks for chlamydia, gonorrhoea, syphilis, and hepatitis are important. These can be done at the same clinic visit as your HIV test.
Kidney Checks
Kidney function is monitored periodically, particularly for people with other health conditions or on medications that may affect kidney health.
Condom Use and Ongoing Prevention
Why Condoms Still Matter
PrEP is extremely effective against HIV, but it offers no protection from other STIs such as chlamydia, gonorrhoea, or syphilis. Condoms remain one of the simplest and most reliable tools for preventing these infections.
Combining PrEP and Condoms
Using both together maximises protection, PrEP covers HIV, condoms cover a wide range of STIs, and both help reduce anxiety around sexual activity.
Building a Personal Prevention Plan
Sexual health is personal. Some will feel secure on PrEP alone, others will prefer combining it with condoms. The key is an informed decision based on your risk level, lifestyle, and comfort with ongoing clinic visits for monitoring.
PEP Vs. PrEP: At a Glance
| Feature | PEP (Post-Exposure Prophylaxis) | PrEP (Pre-Exposure Prophylaxis) |
| Purpose | Emergency HIV prevention after a possible exposure. | Ongoing HIV prevention before potential exposure. |
| When to Start | Within 72 hours of possible exposure. | Before potential exposure, taken daily or event-based. |
| Duration | 28-day continuous course. | Ongoing as long as HIV risk continues. |
| How it Works | Blocks HIV from taking hold in the body after entry. | Maintains drug levels to block HIV if it enters the body. |
| Effectiveness | High if started promptly and taken correctly, but not guaranteed. | Around 99% effect for sexual transmission when taken as prescribed. |
| Who it's For | Anyone with a recent high-risk exposure (sexual, needle-sharing, occupational). | HIB-negative people at ongoing higher risk of exposure. |
| Access in UK | Available at sexual health clinics, hospital A&E departments, some GPs. | Available free via NHS sexual health clinics, some private purchase options. |
| Side Effects | Usually mild and temporary. Can include nausea, headaches, tiredness. | Usually mild and temporary. Regular kidney checks advised for long-term use. |
| Follow-up | HIV testing at baseline, 6 weeks, and 3 months. | HIV testing every 3 months, plus STI and kidney checks. |
| Condoms Still Needed? | Yes. To prevent HIV during and after treatment, and for other STIs. | Yes. PrEP does not protect against other STIs. |
