HIV Prevention • PEP Guide

Is PEP
100% effective?

No, but it works very well when you start early and take every dose. Here’s what the evidence actually shows, and what the small remaining risk depends on.

Medically reviewed by Jude Ozioko, Pharmacist (PCN 024035) • Last reviewed October 2026

Still within 72 hours of exposure? Every hour counts. Order PEP Now →

If you’ve started PEP and you’re wondering whether it’s going to work, that’s a completely natural thing to want to know. The honest answer: it’s very likely to work, but no medicine offers a 100% guarantee. This article explains what drives that small remaining risk, and what you can do to keep it as low as possible.

What the Evidence Shows

Highly effective, but not a precise percentage.

When PEP is started early, ideally within a few hours of exposure, and taken every day for the full 28 days, infection after PEP is uncommon. The cases where PEP has failed are almost always linked to starting late, missed doses, or a new exposure during the course.

You may see websites quote PEP as “over 99% effective”. Be careful with that. It isn’t possible to run the kind of trial that would give PEP an exact figure, the way trials have for PrEP, so nobody can honestly put a precise number on it. What we do know is that PEP works best the sooner it starts and the more consistently it’s taken.

That’s very good news, but the remaining risk doesn’t disappear just because you took the medication. Most of the factors that reduce protection are within your control.

What Reduces Effectiveness

The factors that shrink PEP’s protection.

Starting late. The 72-hour window is the outer limit, not the target. PEP started at hour 60 is less effective than PEP started at hour 4, because every hour of delay gives HIV more time to settle in before the medicine can block it. If you’re within the window, act now.

Missing doses. This is the most preventable cause. PEP is one tablet every day at about the same time. Missed doses lower the drug level in your body and open gaps where the virus can copy itself. Set a phone alarm and treat it as non-negotiable. If you do miss one, see missed a dose of PEP.

Re-exposure during the course. PEP covers one past exposure. If you have unprotected sex again while taking it, it may not be enough to cover a new exposure too. Use condoms throughout your 28 days, and until your 3-month test is negative.

Very high viral load or drug-resistant HIV in the source. You can’t control these, but they’re the reason starting fast matters: the earlier PEP acts, the less impact either one has.

Stopping early is just as risky as missing doses. Feeling fine at day 14 doesn’t mean the virus has been cleared. It means the medicine is doing its job. Complete all 28 days.

After PEP

Testing confirms it worked, not how you feel.

Finishing PEP doesn’t tell you the outcome, and neither do symptoms: early signs of HIV overlap with common illnesses such as malaria and typhoid. Only testing confirms the result.

Test for HIV at 4 to 6 weeks after the original exposure, then take a final test at 3 months. A negative result at 3 months is your confirmation. Don’t skip follow-up testing, however well the course went. Read our full guide on what happens after you finish PEP, and can PEP fail?