Post-Exposure Prophylaxis (PEP): The Urgent HIV Prevention Option
Life doesn’t always give us the luxury of planning ahead. Sometimes events unfold in ways that leave us with questions and, in certain situations, a very real sense of urgency. Picture it: an unplanned sexual encounter without protection, a slip-up in a medical setting, or even an incident involving drug use where equipment is shared. These are the moments that can leave someone wondering if they have been exposed to HIV.
While those thoughts can be overwhelming, there is an option that can significantly reduce the risk - Post-Exposure Prophylaxis, or PEP. This isn’t something to keep in the medicine cabinet “just in case” or to take casually. It’s a highly time-sensitive medical treatment that works only if started quickly and taken exactly as prescribed. Think of it as a kind of emergency fire extinguisher for HIV prevention.
What makes PEP different from other prevention tools is its urgency. You have a narrow window, ideally within hours, and certainly within 72 to act. It is intended as a backup plan when regular prevention methods weren’t used or failed, not as a substitute for them. While PEP can be highly effective, it’s not perfect. The stakes are high enough that starting it late or skipping doses can mean it won’t work.
What is Post-Exposure Prophylaxis (PEP)?
The Basics of PEP
Post-Exposure Prophylaxis is a course of antiretroviral drugs designed to prevent HIV from taking hold in the body after a possible exposure. The treatment usually lasts 28 days and involves taking a combination of medicines each day, at the same time, without missing doses.
The key factor here is timing. PEP only works if it’s started within 72 hours of the potential exposure and the earlier, the better. HIV replicates rapidly in the body once it enters, and the medication aims to block that replication before the virus becomes established.
When PEP Might Be Considered
Doctors or sexual health specialists may recommend PEP in several situations:
- Sexual exposure: Unprotected intercourse with someone known to have HIV, or whose status is unknown but may be at higher risk
- Needle sharing: Using injection equipment that may have been contaminated with HIV
- Occupational exposure: Healthcare or laboratory workers who sustain injuries involving needles or sharp instruments from an HIV-positive patient
- Sexual assault: In many cases, PEP is offered as part of medical care
Not every possible contact with HIV will require PEP. For example, casual contact, kissing, or touching bodily fluids on intact skin does not pose a real risk. The decision is based on whether HIV could realistically have entered the bloodstream.
The Urgency Factor
What makes PEP different from other medicines is that its effectiveness falls sharply the longer you wait to start. This is why people are encouraged to seek help immediately, even if they’re unsure whether their risk is high enough. The assessment itself is quick, and the earlier the treatment begins, the greater the likelihood it will work.
How PEP Works in the Body
The Science Behind It
HIV infects certain immune system cells, particularly CD4 cells, and uses them to make copies of itself. It does this through a multi-step process entering the cell, converting its RNA into DNA, and integrating that DNA into the host’s genetic code. Once integrated, it can produce new virus particles indefinitely.
PEP uses a combination of antiretroviral drugs to block the virus at different stages of this process. Some drugs stop the reverse transcriptase enzyme from working, so HIV’s RNA can’t be converted into DNA. Others block the integrase enzyme, which HIV needs to insert its DNA into the host cell. By hitting the virus from different angles, the treatment makes it much harder for the infection to take hold.
Preventing Establishment of Infection
PEP doesn’t “kill” HIV directly, instead, it buys the immune system time. If the virus is prevented from making new copies long enough, the body’s defences can clear it before it becomes a lifelong infection. But this is only possible if the virus hasn’t already integrated into the host’s DNA, which is why timing is everything.
Why Adherence is Key
Taking the medication exactly as prescribed is essential. Missing even a few doses can allow HIV to start replicating again. Because side effects can be unpleasant, some people are tempted to skip pills, but this significantly reduces the chances of success. Healthcare providers often suggest taking the tablets at a convenient, consistent time each day and using reminders to avoid missed doses.
When and Why PEP is Prescribed
Assessing the Risk
Not all potential exposures are equal. Medical staff will consider factors like:
- The nature of the contact (sexual, needle-sharing, occupational)
- Whether the source person is known to have HIV and their treatment status
- How much bodily fluid was involved and whether it had a realistic route into the bloodstream
A needle-stick injury with fresh blood from an HIV-positive person is very different from a splash on intact skin.
Common Scenarios for PEP Use
- Sexual exposure: Particularly receptive anal or vaginal intercourse without a condom, where HIV risk is highest
- Injection drug use: Sharing needles, syringes, or other drug preparation equipment
- Occupational exposure: Healthcare workers dealing with blood or sharp instruments
- Sexual assault: Often assessed on a case-by-case basis, but PEP is frequently offered
Accessing PEP
PEP is available in the UK from sexual health clinics, hospital A&E departments, and some GP surgeries. However, not all locations keep it on site. If you think you might need it, call ahead or go to a service known to provide it. NHS 111 can direct you to the nearest suitable location.
Side Effects and Considerations
Possible Side Effects
Like many prescription medicines, PEP can cause side effects, though not everyone experiences them and most are manageable. Common issues include nausea, diarrhoea, tiredness, headaches, and sometimes sleep disturbances or vivid dreams. While uncomfortable, these symptoms are usually temporary and resolve once the course is finished.
For most people, these side effects are outweighed by the benefit of preventing HIV. However, it’s worth discussing them upfront so you’re prepared. For example, taking the medication with food can help reduce nausea, and staying well-hydrated can ease headaches.
Safety and Medical Assessment
Before prescribing PEP, a healthcare provider will take a medical history to ensure the chosen drug combination is safe. Certain conditions, such as kidney or liver problems, may mean some medicines are unsuitable. Drug interactions are also considered, so it’s important to be open about any other medications or supplements you take.
Short-term use of antiretroviral medicines is considered safe for most people. Long-term side effects seen in people with HIV on continuous treatment are unlikely to occur with just 28 days of use. Still, it’s important to follow the prescribed plan and attend any recommended check-ups during the course.
Managing the Course
The most important thing after starting PEP is adherence, taking every single dose without fail. Setting alarms, using a pill organiser, or linking the medication to a daily habit (such as brushing teeth at night) can help maintain consistency. If a dose is missed by only a few hours, it should be taken as soon as remembered, but if it’s close to the next dose, skip it and continue as scheduled. Double doses are not advised.
It’s also important to avoid potential HIV exposures while on PEP, as reinfection is still possible. That means using condoms and not sharing needles until follow-up testing confirms you are HIV-negative.
Follow-up Testing and Monitoring
Why Testing Matters After PEP
Completing a 28-day course of PEP is only part of the process. Follow-up HIV testing is essential to confirm whether the treatment was successful. Even when PEP is started promptly and taken correctly, it isn’t guaranteed to work, so testing provides the reassurance or the early detection needed for next steps.
Understanding the Testing Timeline
Testing usually happens at several stages:
- Baseline test: Before starting PEP, to check whether HIV was already present
- 6-week test: Early follow-up to detect any infection that might show up quickly
- 3-month test: Considered the conclusive result for most people
- 6-month test: Occassionally recommended for certain cases, depending on the type of HIV test used and the nature of the exposure
A negative result at 6 weeks is a good sign, but only the later test can confirm you are HIV-negative.
More Than Just HIV Testing
Follow-up appointments are an opportunity for broader sexual health screening. Tests for other STIs such as gonorrhoea, chlamydia, syphilis, and hepatitis are often offered, as these can be transmitted more easily than HIV and aren’t prevented by PEP.
For people who find themselves at risk more than once, healthcare providers may discuss switching from emergency use of PEP to ongoing prevention with PrEP (Pre-Exposure Prophylaxis). This daily or event-based medicine offers continuous protection and can reduce the need for repeated emergency treatments.
Condom Use and Ongoing Prevention
Condoms as a First-Line Defence
Condoms remain one of the most effective ways to prevent HIV and other sexually transmitted infections. They work immediately, without the need for prescriptions, and protect against a wide range of infections that PEP does not cover, including chlamydia, gonorrhoea, and syphilis.
Condoms also provide protection during all types of penetrative sex, not just vaginal intercourse, and when used correctly, their failure rate is low. They are affordable, widely available, and can be discreetly carried for use when needed.
Why PEP Shouldn’t Replace Condoms
PEP is designed for emergencies, situations where protection wasn’t used or failed. It requires urgent medical assessment, a 28-day commitment, and follow-up testing. Relying on PEP as a regular prevention method is neither practical nor healthy, and repeated courses may increase the risk of side effects and drug resistance over time.
In contrast, condoms offer ongoing, in-the-moment protection without the medical complexity. They also protect against infections that are far more easily transmitted than HIV, making them an important part of safer sex strategies.
Building a Sustainable Prevention Plan
The best prevention approach is one that fits your lifestyle and risk level. For some, this means consistent condom use. For others, it could involve a combination of condoms and PrEP, regular STI testing, and honest discussions with partners about sexual health.
PEP has a valuable role, but it should be viewed as the emergency option in a wider toolkit. By keeping condoms accessible and understanding how to use them correctly, many people can avoid situations where PEP would be necessary in the first place.
PEP Quick Facts
- What it is: A 28-day course of HIV medicines taken after a possibile exposure to step the virus taking hold
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When to start: As soon as possible, ideally within a few hours and no later than 72 hours after exposure
- Where to get it: Sexual health clinics, hospital A&E departments, and some GP surgeries (not all stock it)
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Common reasons for use:
- Unprotected sex with someone who is HIV-positive or at higher risk
- Sharing needles or injection equipment
- Occupational exposure in healthcare settings
- Certain cases of sexual assault
- Effectiveness; High when started quickly and taken correctly, but not guaranteed
- Side effects: Nausea, tiredness, diarrhoea, headaches - usually mild and temporary
- Follow-up testing: HIV tests at baseline, 6 weeks, and 3 months (sometimes at 6 months)
- Not a replacement for condoms: PEP is for emergencies, condoms protect against HIV and other STIs in ongoing situations
