PrEP and PEP both prevent HIV, but the timing differs. PrEP (pre-exposure prophylaxis) is medicine HIV-negative people take before possible exposure on an ongoing basis. PEP is the emergency option started after a single possible exposure. PrEP is planned, daily protection; PEP is a short, time-sensitive rescue course.
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 |
What each one is
PrEP stands for pre-exposure prophylaxis. HIV-negative people take it regularly to keep HIV from ever taking hold if they're exposed CDC, Talk PrEP Together. Think of it like a seatbelt you buckle before the drive. The drug stays in your system so that if HIV does get in, it can't establish an infection.
PEP stands for post-exposure prophylaxis. You start it after a single possible exposure — a condom break, a needlestick, an assault — to head off infection before it sets in CDC, Preventing HIV with PEP. PEP is time-sensitive and meant for one-off situations, not for someone who has repeated exposures month after month.
The two are not interchangeable. PrEP is taken on an ongoing basis before exposure; PEP is taken after a single possible exposure. Using one in place of the other leaves real gaps in protection.
The key differences
Timing: before vs after
PrEP is for people who know they may face HIV exposure and want steady, ongoing protection. PEP is for the unexpected, when you didn't plan for it and the clock is running. PEP has to be started as soon as possible after exposure to have a chance of working.
How well they work
Taken as prescribed, PrEP reduces the risk of getting HIV from sex by about 99% and the risk from injection drug use by at least 74%. Those numbers depend on taking it consistently. A pill bottle in the cabinet does nothing.
What they protect against
Both target HIV only. PrEP protects against HIV but not against other STIs — chlamydia, gonorrhea, and syphilis — and not against pregnancy. People on PrEP still need condoms or other contraception for those risks, and they keep testing regularly. You can get tested for the rest while you're on it.
How long until you're protected (PrEP)
PrEP doesn't switch on the moment you swallow the first pill; the medicine has to build up in the tissues it protects, and that ramp-up differs by route of exposure. It takes about 7 days to reach maximum protection for receptive anal sex, and about 21 days for receptive vaginal sex and for injection drug use. If you're starting PrEP, plan for that lead time and use backup protection until you're covered.
PEP vs PrEP at a glance
| PrEP (pre-exposure) | PEP (post-exposure) | |
|---|---|---|
| When you take it | Before possible exposure, on an ongoing basis | After a single possible exposure, as soon as possible |
| Who it's for | HIV-negative people with ongoing risk | HIV-negative people after a one-off exposure |
| Schedule | Daily pill or a periodic shot | A short emergency course |
| Protects against | HIV only — not other STIs or pregnancy | HIV only |
| Best thought of as | A seatbelt buckled before the drive | An emergency brake after the near-miss |
Which one applies to you
Use this simple decision tree. Has a possible exposure already happened — in the last few days? Then PEP is the conversation to have right now, urgently, because it only works in a tight window after exposure. Do you expect ongoing chances of HIV exposure going forward — a partner whose status you don't know or who is living with HIV, condomless sex, or shared injection equipment? Then PrEP is the tool built for you.
If you finish a course of PEP and your risk isn't going away, talk about transitioning onto PrEP so you're not relying on the emergency option again and again.
The practical next step (choosing a PrEP option)
If PrEP is the right fit, there are a few options. Two are daily oral pills:
- Truvada is for people at risk through sex or through injection drug use.
- Descovy is for those at risk through sex only — it's not approved for people assigned female at birth who are at risk through receptive vaginal sex.
- Apretude (cabotegravir) is an injectable PrEP — a shot for people at risk through sex who weigh at least 77 pounds (35 kg), and a good fit for anyone who'd rather not take a daily pill.
If you're weighing the two daily pills, here's a deeper look at truvada vs descovy for prep and which one matches your situation. Whichever you pick, starting PrEP means an HIV test first — you have to be HIV-negative to begin — plus regular check-ins while you're on it. Clinics and telehealth services both prescribe it, and assistance programs exist to cover the cost, so price shouldn't be the thing that stops you.
When to talk to a clinician
Reach out right away if you think you've had a recent HIV exposure, because PEP is time-sensitive and every hour counts. Reach out soon, without the same emergency pressure, if you want to start PrEP because your risk is ongoing. The most common mistake I see is treating PrEP like a morning-after pill. It only works taken on an ongoing schedule, so it can't fix an exposure that already happened. And because it doesn't cover other STIs, staying on PrEP and skipping routine testing leaves a blind spot, so keep testing on schedule. If you're not sure when to screen after a possible exposure, here's when to test after exposure. For couples where one partner is living with HIV, know that earlier hiv treatment can help prevention — an undetectable partner can't transmit the virus sexually, which changes the prevention picture too.