PEP (post-exposure prophylaxis) is emergency HIV medicine you take after a possible exposure to stop the virus from establishing itself. You must start it within 72 hours of the exposure — and the sooner the better, because every hour counts. Once started, PEP is taken daily for 28 days CDC, Preventing HIV with PEP.
sooner is better — every hour counts
taken daily
not a substitute for PrEP
| Item | Value |
|---|---|
| Start within | 72 hours — sooner is better — every hour counts |
| Course | 28 days — taken daily |
| Use | emergencies — not a substitute for PrEP |
| Where | ER / urgent care |
What PEP is and how it works
PEP stands for post-exposure prophylaxis. It's a short, intensive course of the same antiretroviral drugs used to treat HIV, but here the goal is prevention: to keep a recent exposure from turning into a lasting infection. When HIV enters the body, it doesn't infect you instantly — there's a brief window where the virus is still trying to get into your cells and replicate. PEP floods your system with medicine during that window, blocking the virus from copying itself and giving your immune system the upper hand before HIV can take hold.
This is strictly an emergency tool. PEP is for one-off, unexpected situations — not something you take on a schedule. If you find yourself reaching for PEP again and again, that's a signal to switch strategies, which we'll get to below.
How well does PEP work?
PEP is effective when it's started fast and taken correctly, but its protection hinges entirely on two things: how quickly you begin and whether you finish the full course. The clock starts at the moment of exposure, not when you start worrying about it. After 72 hours (3 days), PEP is no longer effective — the virus has had too long to establish itself, and the medicine can't undo that.
Within that 72-hour window, sooner is dramatically better. Starting within hours beats starting on day two, which beats starting at the edge of the deadline. Treat it like a same-day errand, not a wait-and-see decision. And skipping doses or stopping early can let the virus slip through, so the full daily course over 28 days matters as much as starting on time.
Who PEP is for and how to use it
Consider PEP after any recent event where HIV could have entered your body. Common situations include:
- A possible exposure during sex — for example, a condom broke or slipped, or you had sex without a condom with someone whose status you don't know.
- Sharing needles, syringes, or other injection equipment.
- After a sexual assault.
- A possible exposure at work, such as a needlestick injury in a health care setting.
If any of these happened in the last three days, act now. Don't try to figure out the odds on your own at 2 a.m. — that's the clinician's job, and you can sort the details once the medicine is in hand. Once you're prescribed PEP, you take it every day for 28 days. Finishing the whole course is non-negotiable; an early stop can leave the door open for the infection PEP was meant to prevent.
Cost and how to get PEP fast
To start PEP, talk right away to a health care provider, an emergency room doctor, or an urgent care clinician — including after a possible workplace exposure. An ER or urgent care can begin treatment the same day, which matters when your regular doctor's office is closed. The 72-hour clock doesn't pause for business hours, so go to whichever door opens fastest. For specifics on same-day options near you, see our guide on where to get pep fast within 72 hours.
Don't let worry about cost stall you. Ask the clinic or pharmacy about patient-assistance programs, and start the medicine while the paperwork gets sorted out. The single biggest mistake people make with PEP is delaying to figure out payment — every hour spent on logistics is an hour the virus gets to work. Begin treatment first; resolve the billing second.
What PEP does NOT protect against
PEP is built for one job: preventing HIV after a specific, recent exposure. It does nothing for other sexually transmitted infections like chlamydia, gonorrhea, or syphilis, and it isn't ongoing protection — it covers the event that prompted it and nothing more. The moment your 28-day course ends, you have no continued shield against future exposures.
PEP also doesn't replace HIV testing. After a possible exposure you'll still need follow-up testing to confirm your status, and the timing matters — see when to test after exposure so you test at the right moment rather than too early. If you haven't been screened recently for other infections, it's a good time to get tested for the full panel.
How PEP fits with the rest of your prevention
PEP is the emergency brake, not the seatbelt you wear every day. If you have repeated possible exposures — an ongoing relationship with a partner whose status is unknown or who has HIV, or regular needle sharing — relying on PEP each time isn't the right tool. That's what PrEP is for: a daily or scheduled medicine you take before exposure to stay continuously protected CDC, Talk PrEP Together. Ask your provider whether PrEP fits your situation.
Condoms still cut your risk of HIV and other STIs and are worth using alongside any medical prevention. And for partners living with HIV, staying on treatment that keeps the virus undetectable protects partners too — earlier hiv treatment can help prevention explains how that works. PEP, PrEP, condoms, treatment-as-prevention, and regular testing aren't competitors; they're layers, and the strongest protection uses more than one.
When to talk to a clinician
If a possible exposure happened within the last 72 hours, talk to a clinician today — not tomorrow, not after the weekend. There's no need to be certain the exposure was high-risk; bring the facts and let the provider weigh them. The questions worth raising in that visit: whether PEP is right for this exposure, when to come back for HIV testing, whether you should be screened for other STIs, and — if exposures keep happening — whether PrEP is a better long-term fit than repeating PEP.