PEP (post-exposure prophylaxis) is a course of HIV medication taken after a potential exposure to the virus. It works by blocking HIV from replicating and establishing itself in your body during the narrow window between exposure and infection. When started within 72 hours and taken for the full 28-day course, PEP is estimated to be over 80% effective at preventing HIV, with some research suggesting efficacy above 90%.
How PEP Stops HIV at the Cellular Level
When HIV enters your body through a cut, mucous membrane, or sexual exposure, it doesn’t immediately spread everywhere. The virus first infects immune cells near the exposure site, specifically tissue macrophages and dendritic cells. For the next 48 to 72 hours, it replicates locally. If the body can’t contain it at this stage, the virus moves into regional lymph nodes. Full-blown viremia, where HIV is circulating freely in the bloodstream, typically follows within 3 to 5 days of the initial exposure.
PEP takes advantage of this delay. The antiretroviral drugs in a PEP regimen act before the virus integrates its genetic material into your cells’ DNA. This is a critical distinction: once HIV has woven itself into a cell’s DNA, that cell is permanently infected and will produce new copies of the virus for as long as it lives. By flooding the body with drugs that block viral replication before integration happens, PEP can stop HIV from ever gaining a permanent foothold.
The 72-Hour Window
PEP must be started within 72 hours (3 days) of a potential HIV exposure. Every hour counts. Ideally, you should begin within 24 hours of the exposure, since earlier treatment gives the drugs a better chance of intercepting the virus before it spreads beyond the initial infection site. The CDC states that PEP is unlikely to be effective if started more than 72 hours after exposure.
This urgency is directly tied to the biology described above. Once HIV has replicated in lymph nodes and entered the bloodstream, antiretrovirals can slow but not eliminate it. The goal of PEP is to catch the virus while it’s still local and vulnerable.
What You Take and For How Long
A PEP course lasts 28 days. You take a combination of antiretroviral drugs, typically as a single pill or a small number of pills each day. The 2025 CDC guidelines recommend a three-drug combination as the preferred regimen, combining two types of drugs that each block HIV replication through different mechanisms. An older combination that was preferred under the 2016 guidelines is still considered acceptable, so your provider may prescribe either depending on availability and your health profile.
Completing all 28 days matters. In a review of 15 studies involving over 2,200 people who completed a full PEP course, only one case of HIV was attributed to actual PEP failure. The other infections that occurred were linked to people who continued high-risk behavior after PEP ended, didn’t take the medication as prescribed, or started treatment too late.
Side Effects During Treatment
Modern PEP regimens are generally well tolerated compared to older HIV medications. The most common side effects are gastrointestinal: nausea, diarrhea, and fatigue. These symptoms tend to be mild and often improve after the first week or so as your body adjusts. Some people experience headaches or trouble sleeping. Serious side effects are uncommon with current drug combinations, which were updated partly based on improved safety and tolerability data.
If side effects become difficult to manage, contact your prescribing provider rather than stopping the medication on your own. Skipping doses or quitting early reduces PEP’s effectiveness.
Where to Get PEP Quickly
Because timing is critical, PEP is available through several access points. You can go to an emergency room, an urgent care clinic, or contact your regular healthcare provider. Emergency rooms stock PEP and can prescribe it on the spot after evaluating your exposure. In some states, including California, New York, Colorado, Illinois, Oregon, and several others, pharmacists can prescribe PEP directly, which can speed up access.
If you’re unsure where to go, HIV.gov offers a services locator tool to find PEP providers near you. Don’t wait for a regular appointment if one isn’t available within hours. The difference between starting PEP at hour 12 versus hour 60 could affect whether it works.
Follow-Up Testing After PEP
Finishing the 28-day course isn’t the final step. You’ll need follow-up HIV testing to confirm the treatment worked. Your provider will schedule tests at intervals after you complete PEP, since HIV antibodies take time to appear on standard tests. A negative result too early could be misleading, so it’s important to return for testing at the timepoints your provider recommends rather than relying on a single early test.
If you find yourself needing PEP more than once, your provider will likely discuss PrEP (pre-exposure prophylaxis), a daily or on-demand medication for people at ongoing risk of HIV exposure. PrEP is taken before potential exposure rather than after, removing the pressure of the 72-hour window entirely.