PrEP and PEP are two HIV prevention strategies that use antiretroviral medications to stop the virus before it can establish an infection. PrEP is taken before a potential exposure, while PEP is taken after one. They differ in timing, duration, who they’re for, and how they’re prescribed, but both work by blocking HIV’s ability to replicate inside your body.
PrEP: Prevention Before Exposure
PrEP stands for pre-exposure prophylaxis. It’s a medication taken on an ongoing basis by people who don’t have HIV but are at higher risk of getting it. When taken consistently, PrEP reduces the risk of getting HIV from sex by about 99%. For people who inject drugs, it lowers the risk by at least 74%.
PrEP works by keeping enough of the drug in your body so that if HIV enters through sex or needle sharing, the virus can’t complete its life cycle. It gets blocked before it can copy itself and spread to other cells. The key word here is “consistently.” Those high protection numbers depend on taking the medication as prescribed. Skipping doses lowers the drug levels in your body and reduces protection.
PrEP comes in two forms. The first is a daily pill. The second is a long-acting injectable given by a healthcare provider every two months after an initial loading period. The injectable option can be a good fit if remembering a daily pill is difficult or if you prefer not to keep medication at home.
PEP: Prevention After Exposure
PEP stands for post-exposure prophylaxis. It’s an emergency course of antiretroviral medication for someone who may have just been exposed to HIV. Common scenarios include unprotected sex with a partner whose HIV status is unknown or positive, a condom breaking, sharing needles, or a needle stick injury at work.
The critical detail with PEP is the clock. You must start PEP within 72 hours of exposure, and sooner is significantly better. Research modeling the effectiveness of PEP found that over 90% efficacy could be achieved when the medication was started within 48 hours and taken for the full course. Waiting longer sharply reduces how well it works. If more than 72 hours have passed, PEP is generally not prescribed because the window of opportunity has closed.
PEP is a 28-day course. You take the medication every day for four weeks and then stop. It’s not meant for repeated or ongoing use. If you find yourself needing PEP more than once, that’s a strong signal that PrEP may be a better fit for your situation.
Key Differences at a Glance
- Timing: PrEP is taken before any exposure occurs. PEP is started after a specific exposure, within 72 hours.
- Duration: PrEP is ongoing, taken for as long as you remain at risk. PEP is a fixed 28-day course.
- Purpose: PrEP is a routine prevention tool. PEP is emergency prevention.
- Form: PrEP is available as a daily pill or a bimonthly injection. PEP is a daily pill regimen only.
Who PrEP Is For
PrEP is appropriate for anyone who is HIV-negative and has an ongoing chance of exposure. That includes people with an HIV-positive sexual partner, people who don’t consistently use condoms with partners whose status they don’t know, people who have had a recent sexually transmitted infection, and people who share injection equipment. The current CDC guidance is broad: if you’re at risk, PrEP should be discussed with a provider regardless of age, gender, or sexual orientation.
Who PEP Is For
PEP is for anyone who is HIV-negative and has had a specific, recent exposure. This could be sexual contact, a needle stick, or any situation where mucous membranes, broken skin, or punctured skin came into contact with body fluids from someone who has or might have HIV. It’s also used in occupational settings, such as healthcare workers who experience accidental needle injuries. A healthcare provider evaluates the nature of the exposure and decides whether to prescribe the 28-day course.
What to Expect: Testing and Side Effects
Before starting PrEP, you’ll need an HIV test to confirm you’re negative. Starting PrEP while unknowingly HIV-positive can lead to drug resistance, which would limit future treatment options. For the oral pill forms, kidney function is also checked before prescribing, since the medication can cause small, usually reversible decreases in kidney function. If you’re on oral PrEP long term, kidney monitoring continues at least once a year, or every six months if you’re over 50 or started with lower kidney function.
The injectable form of PrEP does not require kidney monitoring. The most common side effect with injections is soreness, swelling, or tenderness at the injection site. These reactions are typically mild to moderate and resolve quickly.
PEP also requires an HIV test at the start, and follow-up testing after the 28-day course to confirm the medication worked. Side effects during the four-week PEP course can include nausea and fatigue, but these are temporary and manageable for most people.
How They Work Together
PrEP and PEP aren’t competing options. They serve different moments in the same goal of staying HIV-negative. Someone on PrEP who takes their medication consistently doesn’t need PEP, because the protection is already in place. But someone who isn’t on PrEP and has an unexpected exposure can use PEP as a safety net. After completing a PEP course, many providers will discuss transitioning to PrEP if the person’s risk of future exposure is ongoing. Moving from a reactive strategy to a proactive one is almost always more effective and less stressful than relying on emergency medication after the fact.