PEP (post-exposure prophylaxis) is a 28-day course of HIV medicine you start within 72 hours of a possible exposure to keep the virus from taking hold. The sooner you begin, the better it works; in the original study it cut HIV seroconversion by about 81% CDC, PEP. Treat it as a same-day emergency.
in 2023
≈723,000 — U=U
| Item | Value |
|---|---|
| New diagnoses | 38,800 — in 2023 |
| Living with HIV | 1.12 million |
| Virally suppressed | ~65% — ≈723,000 — U=U |
| On PrEP | 381,000 |
What PEP is and how it works
PEP uses the same kinds of antiretroviral drugs people take to treat HIV, but here the goal is prevention. HIV infects immune cells and copies itself fast. Flood the body with medicine before the virus establishes a lasting infection and you can stop those first rounds of replication and clear the exposure before it becomes permanent. The window is short. The medicines have to be on board while the virus is still trying to gain a foothold, so every hour counts after a possible exposure CDC, About HIV.
HIV is only transmitted through specific fluids — blood, semen, vaginal fluid, rectal fluid, and breast milk — and through anal or vaginal sex, sharing needles or injection equipment, or from parent to baby in pregnancy, birth, or breastfeeding. It does not spread through saliva, kissing, casual contact, surfaces, food, water, or insects, and it doesn't survive long outside the body CDC, How HIV Spreads. So a real exposure worth treating means a needlestick, a condom break, sexual assault, or condomless sex with a partner whose status is positive and not suppressed.
The 72-hour clock: timing is everything
PEP must be started within 72 hours (3 days) of the exposure, and ideally within hours. After that window the drugs are far less likely to prevent infection, and PEP is no longer recommended. If you think you've been exposed, don't wait for symptoms and don't wait to test — go to an emergency room, urgent care, or sexual-health clinic right away and ask for PEP by name.
The clock is tight for a reason. The virus replicates quickly once it enters the body, and acute HIV — the first few weeks — is when the viral load peaks and the virus spreads internally most aggressively. PEP has to interrupt that early surge. Showing up on day four with flu-like symptoms is too late for prevention, and at that point you need urgent testing instead.
What taking PEP is actually like
PEP is daily pills taken for 28 days straight. Finishing the full course matters, because stopping early leaves the door open for the virus to establish itself. Some people get nausea, fatigue, or headache in the first days. Modern regimens are generally well tolerated, and if side effects are rough, your clinician can adjust the medicine rather than have you quit.
- Go immediately — emergency department, urgent care, or a sexual-health clinic. Don't book a routine appointment days out.
- Ask for PEP by name and tell them when the exposure happened, down to the hour if you can.
- Bring details about the exposure (type of contact, and the other person's HIV status and treatment if known); it helps choose the regimen.
- Take every dose for the full 28 days, even if you feel fine.
- Plan the follow-up HIV tests your clinician schedules. A negative result isn't conclusive until the testing window has passed.
Cost and access can feel like a barrier in the moment, but don't let it stop you from showing up. Many emergency departments stock starter doses, and assistance programs exist for the full course. Start on time and figure out coverage afterward.
PEP vs PrEP: don't confuse them
This distinction trips people up. PEP is the emergency brake — you use it after a single possible exposure, and it's a one-time 28-day course. PrEP (pre-exposure prophylaxis) is ongoing protection for people who have repeated exposure through sex or injection drug use, and it works far better when used consistently because the medicine is already in your system before any exposure CDC, PrEP. PEP doesn't replace PrEP or condoms. If you find yourself needing PEP more than once, talk to a clinician about going on PrEP.
| PEP | PrEP | |
|---|---|---|
| When you start | After a possible exposure, within 72 hours | Before exposure, taken on an ongoing basis |
| Who it's for | Emergencies — one-off risk events | People with ongoing risk from sex or injection drug use |
| How long | 28-day course, then done | Continuous, with regular follow-up visits |
| How well it works | Cut seroconversion ~81% in the original study when started fast | Reduces sexual HIV risk ~99% when taken as prescribed |
| Options | A short antiretroviral regimen | Daily Truvada or Descovy; injectable cabotegravir (Apretude) every 2 months after two starter doses |
A couple of PrEP details worth knowing if you transition from PEP: Descovy isn't approved for people at risk through receptive vaginal sex or for people who inject drugs, while Truvada covers all those routes. PrEP also requires a confirmed HIV-negative test before you start and at follow-up visits — every 3 months for oral PrEP, every 2 months for the injectable — because starting PrEP with an undiagnosed infection risks drug resistance. The newest research is striking: in the PURPOSE 1 trial, twice-yearly injectable lenacapavir produced zero infections among women WHO, lenacapavir.
After PEP: testing and what comes next
PEP doesn't end the story. You'll need follow-up HIV testing to confirm the exposure didn't lead to infection. Test windows vary by type: a nucleic-acid test can detect HIV roughly 10–33 days after exposure, a 4th-generation antigen/antibody lab test about 18–45 days, and antibody or rapid tests about 23–90 days CDC, HIV Testing. A negative result is only conclusive after the window has passed with no further exposure. For a fuller breakdown, see hiv testing and the guide on when to test after exposure.
Watch your symptoms in those first weeks. About 90% of people get flu-like symptoms two to four weeks after infection — fever, sore throat, swollen glands, rash — exactly when the viral load peaks and the virus is most contagious StatPearls, HIV. But many people have none, and the symptoms look like any ordinary virus. Only a test can confirm or rule out HIV. If you develop symptoms after a risk, get tested urgently rather than guessing.
If a test is positive: treatment is also prevention
If PEP doesn't prevent infection, or you learn you already had HIV, the outlook today is good. Modern antiretroviral therapy doesn't cure HIV — the virus persists in latent reservoirs and rebounds if treatment stops — but it controls the virus so well that a 20-year-old who starts early can expect a life expectancy approaching that of the general population Lancet HIV. A person who takes hiv treatment as prescribed and stays virally suppressed will not transmit HIV to sex partners. That's the U=U principle, backed by trials in which mixed-status couples logged over 125,000 condomless sex acts with zero linked transmissions while the positive partner was undetectable PARTNER.
Telling partners is part of this, and it's easier with a plan and support — see our guidance on disclosure of hiv status. If you want a confidential starting point for any of this, you can get tested.
When to see a clinician — urgently
- You think you were exposed in the last 72 hours — go now, today, and ask for PEP. This is the one scenario where hours matter.
- You've had a needlestick, condom failure, sexual assault, or condomless sex with a partner whose HIV status is positive and not suppressed or simply unknown.
- You needed PEP and you're past the 28-day course — book follow-up testing and a conversation about whether PrEP fits your life.
- You develop flu-like symptoms after a possible exposure. Get tested urgently.
- You find yourself reaching for PEP repeatedly. Switch to ongoing protection with PrEP.