If you’ve already been exposed to HIV, what you need is PEP (post-exposure prophylaxis), not PrEP (pre-exposure prophylaxis). PEP is a 28-day course of HIV medications that must be started within 72 hours of exposure. Every hour matters, so if you haven’t started yet, seek care immediately at an emergency room or urgent care clinic.
The two terms sound similar and are easy to mix up, but they serve very different purposes. Here’s what you need to know about both.
PEP Is What You Take After Exposure
PEP stands for post-exposure prophylaxis. It’s an emergency medication for people who may have been exposed to HIV through sex, needle sharing, or occupational contact (like a needlestick). You take a combination of three HIV medications every day for exactly 28 days.
The critical window is 72 hours (3 days) from the moment of exposure. Starting PEP after 72 hours is unlikely to prevent infection. The sooner you begin, the more effective it is, so don’t wait to see if symptoms appear or try to assess your risk on your own. Go to an emergency department or sexual health clinic and tell them you need PEP.
PrEP Is What You Take Before Exposure
PrEP stands for pre-exposure prophylaxis. It’s designed for people who don’t have HIV but face ongoing risk, such as having a sexual partner who is HIV-positive or frequently having condomless sex. PrEP comes as a daily pill or as an injection given every two or six months. It’s a long-term prevention strategy, not an emergency response.
PrEP taken after an exposure won’t work the way PEP does. PrEP uses fewer medications (typically two drugs instead of three), and it needs time to build up protective levels in your body before an exposure happens. If the exposure already occurred, PEP is the correct choice.
What the 28-Day PEP Course Looks Like
Once prescribed, you’ll take your PEP medications once or twice daily (depending on the regimen) for the full 28 days. Completing the entire course is essential. Stopping early, skipping doses, or taking pills inconsistently reduces the medication’s ability to prevent the virus from establishing itself in your body.
Some people experience side effects during the course, most commonly nausea, fatigue, and headaches. These tend to be mild and often improve after the first week or two. Side effects can be uncomfortable, but they’re not a reason to stop taking the medication. If side effects are severe enough that you’re considering stopping, contact your prescribing provider so they can adjust your regimen rather than having you quit.
Follow-Up Testing After PEP
Finishing the 28-day course isn’t the last step. You’ll need HIV testing at specific intervals to confirm the medication worked. The standard schedule is testing at baseline (when you start PEP), at 4 weeks after exposure, and again at 12 weeks after exposure. A negative result at 12 weeks reliably rules out HIV infection from that exposure. Testing at six months is no longer recommended as part of the standard follow-up.
During this testing window, use condoms and take precautions to avoid new exposures. If your test results come back positive at any point, you’ll be connected with HIV treatment right away.
Switching From PEP to PrEP for Ongoing Protection
If the exposure that brought you to PEP reflects an ongoing pattern of risk, your provider will likely talk with you about starting PrEP once your 28-day PEP course is finished. PEP is designed for one-time emergencies. It’s not meant to be used repeatedly because it involves a more intensive drug regimen with more side effects.
Transitioning to daily PrEP after completing PEP gives you continuous protection going forward. Your provider can prescribe PrEP as soon as your PEP course ends, so there’s no gap in coverage. If you know your risk isn’t going away, this conversation is worth having before your 28 days are up.
On-Demand PrEP Is Not the Same as PEP
You may have heard about “on-demand” or “2-1-1” PrEP, which involves taking pills around the time of a planned sexual encounter. This method requires taking two pills 2 to 24 hours before sex, a third pill 24 hours after the first dose, and a fourth pill 48 hours after the first dose. It has been studied as a prevention option for men who have sex with men.
This is still a before-exposure strategy. The loading dose of two pills needs to be taken hours before sex occurs. If the exposure has already happened and you didn’t take those pills in advance, on-demand PrEP will not protect you. You need PEP.
The Bottom Line on Timing
If exposure happened less than 72 hours ago: get PEP now. Take it for 28 days without missing doses. Get tested at 4 weeks and 12 weeks. If you face ongoing risk, talk to your provider about transitioning to PrEP for long-term protection. If more than 72 hours have passed since the exposure, PEP is unlikely to be effective, but you should still see a healthcare provider for HIV testing and to discuss your options going forward.