PrEP is daily (or scheduled) medicine HIV-negative people take before possible exposure to prevent infection; PEP is an emergency course started after a single possible exposure. PrEP is ongoing prevention, while PEP is a short rescue treatment that must begin within about 72 hours. They're not interchangeable.
risk reduction, taken as prescribed
daily Truvada/Descovy or the Apretude injection
not other STIs or pregnancy
| Item | Value |
|---|---|
| From sex | ~99% — risk reduction, taken as prescribed |
| From injection use | ≥74% |
| Forms | pill or shot — daily Truvada/Descovy or the Apretude injection |
| Protects against | HIV only — not other STIs or pregnancy |
PrEP vs PEP: the bottom-line difference
The simplest way to keep these straight is timing. PrEP (pre-exposure prophylaxis) is something you take consistently when you expect ongoing risk — think of it like a seatbelt you wear every drive, not after a crash. PEP (post-exposure prophylaxis) is the emergency option you start fast after one possible exposure, like a condom break or a needlestick. One prevents; the other is damage control after the fact.
Both use anti-HIV medicines, and both are about people who are HIV-negative. The mistake I see most often is someone treating PrEP as a morning-after pill — that's not how it works. PrEP only protects when it's already in your system on a schedule CDC, Talk PrEP Together. If exposure already happened and you're not on PrEP, the question isn't PrEP — it's whether you need PEP right now CDC, Preventing HIV with PEP.
What each one actually is
PrEP (pre-exposure prophylaxis)
PrEP is medicine an HIV-negative person takes before any possible exposure to keep the virus from establishing infection. It's prevention, not treatment — you take it because you're at risk going forward, not because you've been exposed. Taken as prescribed, it cuts the risk of getting HIV from sex by about 99%, and the risk from injection drug use by at least 74%.
PrEP comes as a daily pill or, for some people, a long-acting shot. What it does not do is matter just as much: PrEP protects only against HIV. It won't stop chlamydia, gonorrhea, or syphilis, and it does nothing for pregnancy. That's why people on PrEP still need condoms for other infections and routine screening — more on that below.
PEP (post-exposure prophylaxis)
PEP is the emergency course you start after a single possible exposure to HIV — unprotected sex with someone whose status you don't know, a condom failure, sharing needles, or a sexual assault. It's a combination of anti-HIV medicines taken for a set stretch of weeks to stop the virus before it takes hold.
Timing is everything with PEP. It has to be started as soon as possible after exposure, and there's a hard window — it's meant to begin within 72 hours, and sooner is better. After that window, it's no longer expected to work. PEP is a one-off response to one event, not something you stay on. If you keep facing risk, the right long-term move is to switch to PrEP instead of reaching for PEP again and again.
The key differences
Timing: before vs after
This is the dividing line. PrEP goes in your body before exposure and stays there on an ongoing basis. PEP starts after exposure and races a clock. If you're already on PrEP and taking it correctly, you generally don't need PEP for a routine exposure.
Ongoing vs one-time
PrEP is a regimen you maintain for as long as you're at risk, with regular clinical check-ins. PEP is a single short course tied to one specific exposure. Using PEP repeatedly as your only strategy is a sign you'd be better protected by PrEP.
How well each works — and how fast PrEP kicks in
PrEP's protection isn't instant. It needs time to build to full levels in the tissues that matter, and that timeline depends on the type of exposure: roughly 7 days for receptive anal sex, and about 21 days for receptive vaginal sex and for injection drug use. That lag is exactly why PrEP can't double as morning-after protection — start it well before you'll need it.
PrEP vs PEP at a glance
| PrEP | PEP | |
|---|---|---|
| When you take it | Before possible exposure, ongoing | After a single possible exposure |
| Time-sensitive? | Start ahead of risk; protection builds over days | Must start within 72 hours — sooner is better |
| Who it's for | HIV-negative people with ongoing risk | HIV-negative people after one exposure |
| Duration | As long as risk continues | A short course of weeks, then stop |
| How effective | ~99% from sex; at least 74% from injection drug use, as prescribed | Effective when started promptly within the window |
| Protects against | HIV only — not other STIs or pregnancy | HIV only |
Which one applies to you?
Start with one question: has a possible exposure already happened? If yes, and it was within the last 72 hours, this is a PEP situation — call a clinic, urgent care, or an ER tonight, don't wait. If no, and you expect ongoing risk (a partner whose status you don't know, multiple partners, or injection drug use), PrEP is the tool built for you.
If you're choosing PrEP, the next decision is which kind. The options:
- Truvada (daily pill) — for people at risk through sex or through injection drug use.
- Descovy (daily pill) — for people at risk through sex only; it's not for people assigned female at birth who are at risk through receptive vaginal sex.
- Apretude (cabotegravir injection) — a shot for people at risk through sex who weigh at least 77 pounds (35 kg), and a good fit if a daily pill isn't realistic for you.
Picking between the two pills comes down to your risk route and a few details worth talking through with your clinician — I walk through the trade-offs in truvada vs descovy for prep.
The practical next step
Starting PrEP begins with an HIV test, because PrEP is only for people who are HIV-negative — and you'll have regular check-ins while you're on it to recheck that and screen for other infections. You can get PrEP through a regular clinic or through telehealth, and cost-assistance programs exist to make it affordable or free.
Because PrEP doesn't cover other STIs, staying on it means staying on top of screening too. If you're on PrEP, build routine testing into the plan — get tested on the schedule your clinician sets. And if you're trying to time a test around a specific encounter, here's when to test after exposure.
One bigger-picture point: prevention isn't only about HIV-negative people. When someone living with HIV starts treatment early and reaches an undetectable viral load, they can't pass the virus on through sex — so earlier hiv treatment can help prevention for everyone, alongside PrEP and condoms.
When to talk to a clinician
Reach out promptly if a possible exposure just happened and you're within the 72-hour PEP window — that's a same-day call. Talk to a clinician about PrEP if you have ongoing risk and want steady protection, if you've needed PEP more than once, or if you're already on PrEP and want to switch from a daily pill to the shot. And if you're on PrEP but have new symptoms or a new partner, that's a good moment to check in and update your testing plan.