HIV Prevention • PrEP Evidence

How effective is PrEP
at preventing HIV?

Taken daily, PrEP reduces your HIV risk by up to 99%. Here’s what that number actually means, and what causes the rare cases where it doesn’t work.

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

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PrEP is one of the most effective HIV prevention tools ever made available. When you take it every day as prescribed, it works. But “up to 99%” is a conditional number, it comes with a very important condition. This article explains what drives it and what can shrink it.

The Numbers

What the evidence actually shows.

~99%

Risk reduction for HIV from sex, with consistent daily use

~74%

Risk reduction for HIV from injecting drugs, with consistent daily use

These numbers come from multiple large clinical trials with thousands of participants across different countries. They’re not projections or estimates, they’re observed outcomes in real people taking PrEP correctly. The figures are consistent and have been replicated across studies.

The key phrase is “with consistent daily use.” In the same studies, when people took PrEP less regularly, the protection dropped, sometimes dramatically. The medicine works. What varies is whether the person takes it consistently enough for it to do its job.

Why Daily Dosing Matters So Much

PrEP only works if it’s already in your body when HIV arrives.

Here’s the thing about how PrEP works: it needs to be built up in your body’s tissues before an exposure happens. It can’t work after the fact. That’s what PEP is for. PrEP is a pill you take every day so that the protection is already there, ready, whenever it’s needed.

The reason the timeline to full protection differs between anal sex (about 7 days) and vaginal sex (about 21 days) is that the tissues involved absorb and hold the medicine at different rates. But once the levels are established through daily use, they stay at protective levels as long as you keep taking it. Skip doses, and those levels start to drop.

When PrEP Fails

Why rare breakthrough infections happen.

Missing doses regularly. This is the most common reason by far. Inconsistent dosing drops protection levels in the body, and that gap is when HIV can get through. One missed dose here and there won’t destroy your protection. But missing doses repeatedly over days is a different story.

Starting PrEP too late. If HIV had already entered the body before PrEP reached protective levels, the window for prevention was already closed.

Starting without knowing you’re already positive. This is why getting an HIV test before starting PrEP is non-negotiable. Starting PrEP when you already have HIV, without knowing, risks creating a drug-resistant strain that becomes much harder to treat.

PrEP only protects against HIV, nothing else. Gonorrhoea, syphilis, chlamydia, herpes, PrEP doesn’t cover any of these. Condoms do. The two can be used together without any problem.