PrEP vs PEP: what’s the difference, and which do you need?

PrEP and PEP sound almost identical, and both are ways of using HIV medication to stay HIV-negative — but they work at opposite ends of a situation. One is planned protection you take before any risk; the other is an emergency measure you start after a possible exposure. Getting the difference clear matters, because using the wrong one at the wrong time leaves you unprotected. Here’s how each works, the different ways PrEP can be taken, and how to know which you need.

The quick difference

The simplest way to remember it is in the names. PrEP is pre-exposure prophylaxis — taken before exposure to prevent HIV. PEP is post-exposure prophylaxis — taken after a possible exposure to stop an infection taking hold. PrEP is an ongoing, planned strategy for people with regular risk. PEP is a one-off emergency treatment, and it’s time-critical: it has to be started within 72 hours. Both are highly effective when used correctly, and neither is “better” — they’re built for different situations.

What is PrEP?

PrEP is HIV-prevention medication taken by people who are HIV-negative to protect themselves in advance. Taken as prescribed, it’s remarkably effective — reducing the risk of getting HIV through sex by around 99%. It’s recommended for anyone at ongoing risk, including gay and bisexual men, trans and gender-diverse people, heterosexual people with higher-risk partners, and people who inject drugs.

Because PrEP prevents only HIV, it’s paired with regular check-ups: an HIV test and a sexual health screen roughly every three months, so that it stays safe and effective. At Clinic365 this is all handled by telehealth — see PrEP online.

PrEP dosing: daily, on-demand and periodic

One of the most common points of confusion is that PrEP can be taken in more than one way. There isn’t a single “correct” schedule — the right one depends on who you are and how often you’re at risk.

Daily PrEP

The standard, most widely suitable approach: one tablet every day. Daily dosing keeps protective drug levels constant, so you’re covered regardless of when sex happens. It’s the only method recommended for everyone at risk, including people who have receptive vaginal sex, because protection doesn’t depend on planning ahead.

On-demand PrEP (the “2-1-1” method)

On-demand, or event-driven, PrEP is taken only around specific occasions of sex, using a schedule known as 2-1-1: two tablets 2 to 24 hours before sex, one tablet 24 hours after the first dose, and one more 24 hours after that. It suits people who have sex less frequently or can anticipate it. Importantly, on-demand dosing is recommended only for cisgender men who have sex with men. It is not recommended for people having receptive vaginal sex, because the medication takes longer to reach protective levels in vaginal tissue and the evidence for event-based dosing there isn’t established — daily PrEP is the right choice in that case.

Periodic or “holiday” PrEP

Some people don’t need PrEP all year round — risk might cluster around a holiday, a new relationship, or certain events. “Periodic” PrEP simply means using daily PrEP during those higher-risk stretches and stopping in between, with a doctor’s guidance on how to start up and wind down safely. It’s not a separate pill, just a way of matching PrEP to when you actually need it.

Long-acting injectable PrEP

For people who’d rather not take a daily tablet, long-acting injectable PrEP is a newer option: an injection given every couple of months instead of oral pills. It offers the same goal — steady protection without having to remember a daily dose — and suits people who find pills hard to keep up with. It’s not right for everyone, but it’s worth asking about; you can read more on our injectable PrEP page.

What is PEP?

PEP is the emergency option. If you think you’ve been exposed to HIV — a condom broke, you had sex without protection with someone whose status you don’t know, or there was another risk — PEP is a course of HIV medication that can stop the virus establishing itself, if you start it fast.

Two things about PEP are non-negotiable:

• Timing: it must be started within 72 hours of the exposure, and the sooner the better — every hour counts. After 72 hours it’s no longer effective.
• Duration: it’s taken every day for 28 days, and finishing the full course matters.

Because it’s time-critical, PEP is treated as urgent. It’s available through sexual health clinics, many GPs, and hospital emergency departments after hours. If you need it, see emergency PEP or present to an emergency department without delay.

Which one do you need?

It comes down to timing and how often you’re at risk:

• If you have ongoing or predictable risk and want to stay protected, PrEP is the answer — planned, reliable, and taken in whichever schedule fits your life.
• If you’ve already had a possible exposure in the last 72 hours and you’re not on PrEP, you need PEP now — this is an emergency, not something to book for next week.

There’s also a natural link between the two: if you find yourself needing PEP more than once, that’s a strong sign PrEP would serve you better. Many people start PrEP straight after finishing a course of PEP, moving from emergency cover to steady, planned protection. A doctor can bridge you smoothly from one to the other.

A key limit: PrEP and PEP only cover HIV

This is easy to miss. Both PrEP and PEP protect against HIV and nothing else — they do not prevent chlamydia, gonorrhoea, syphilis or other STIs. That’s why regular STI testing stays part of being on PrEP, and why condoms still have a role. For reducing some bacterial STIs, a separate approach called DoxyPEP exists, and it’s worth understanding how that fits alongside HIV prevention. If you’re not sure what testing you should be doing, our guide to a full STI screen lays it out.

How to get started

Both PrEP and PEP require a prescription and a quick check that they’re right and safe for you (including a current HIV test). PrEP can be arranged calmly and in advance by telehealth, with ongoing three-monthly reviews. PEP is urgent — if you’re within the 72-hour window, act today.

The bottom line: PrEP is planned protection before risk, available as a daily tablet, an on-demand 2-1-1 course for eligible men, a periodic strategy, or a long-acting injection. PEP is emergency protection after a possible exposure, started within 72 hours and taken for 28 days. Knowing which is which — before you ever need it — is one of the simplest ways to stay in control of your sexual health.