Post-Exposure Prophylaxis (PEP)
Key Points
- Post-exposure prophylaxis (PEP) refers to a short course (28 days) of HIV medicines that are taken after a possible exposure to prevent HIV infection.
- PEP must be started within 72 hours after a possible exposure to HIV. The sooner PEP is started after a possible HIV exposure, the better. Every hour counts.
- Although PEP effectively reduces the risk of HIV when taken correctly, it should only be used in emergencies and cannot replace regular prevention methods like pre-exposure prophylaxis (PrEP).
What is PEP?
PEP stands for post-exposure prophylaxis. The word “prophylaxis” means to prevent or control the spread of an infection or disease. PEP refers to the use of HIV medicines to prevent HIV infection within 72 hours (3 days) after a possible exposure.
PEP should be used only in emergency situations and is not meant for regular use by people who may be exposed to HIV frequently. PEP is not intended to replace the regular use of other HIV prevention methods, such as consistent and proper use of condoms during sex or pre-exposure prophylaxis (PrEP).
PrEP is different than PEP, in that people at risk for HIV take a specific HIV medicine daily or an injection every two months to prevent getting HIV before an exposure occurs.
For more information, see the HIVinfo fact sheets on The Basics of HIV Prevention and Pre-Exposure Prophylaxis (PrEP).
Who should consider taking PEP?
PEP may be prescribed for people who do not have HIV or who do not know their HIV status, and who in the last 72 hours:
- May have been exposed to HIV during sex
- Shared needles or other equipment (works) to inject drugs
- Were sexually assaulted
- May have been exposed to HIV at work (occupational exposure)
If you think you were recently exposed to HIV, talk to your health care provider or an emergency room doctor about PEP immediately.
When should PEP be started?
PEP must be started within 72 hours (3 days) after a possible exposure to HIV. The sooner PEP is started after a possible HIV exposure, the better. Every hour counts. According to the Centers for Disease Control and Prevention (CDC), PEP will most likely not prevent HIV infection if it is started more than 72 hours after a person is exposed to HIV.
If you are prescribed PEP, you will need to take the HIV medicines every day for 28 days.
What HIV medicines are used for PEP?
The CDC provides guidelines on recommended HIV medicines for PEP. The CDC guidelines include recommendations for specific groups of people, including adults and adolescents, children, pregnant women, and people with kidney problems.
Your health care provider or emergency room doctor will work with you to determine which medicines to take for PEP.
How well does PEP work?
PEP is effective in preventing HIV when it is taken correctly, but it is not 100% effective. The exact effectiveness of PEP is difficult to measure, but observational research suggests that PEP can reduce the risk of getting HIV by more than 80%.
The effectiveness is highly dependent on adherence (taking PEP every day for 28 days) and, in the case of non-occupational exposure, no additional exposures to HIV. Effectiveness is likely much higher than 80% if PEP is used consistently and correctly, as prescribed.
While taking PEP, it is important to keep using other HIV prevention methods, such as consistently and properly using condoms with sex partners and using only new, sterile needles and syringes if a person injects drugs. HIV PEP does not protect against other sexually transmitted infections (STIs). Using condoms will reduce your risk of HIV and will also help protect against other STIs.
Does PEP cause side effects?
The HIV medicines used for PEP may cause side effects in some people. The common side effects are generally mild and may include diarrhea, nausea, tiredness, and headache, and can be treated or will subside on their own. Less commonly, PEP medicines can cause more serious side effects such as liver problems or lactic acidosis that may require medical attention.
Before taking PEP, be sure to consult your health care provider about any potential side effects cause by PEP medicines. If you are taking PEP, talk to your health care provider if you have any side effects that bother you or that do not go away.
What should you do after taking PEP?
Your next steps after taking PEP will depend on whether the medicine effectively prevented HIV infection.
If you test negative for HIV after a full course of PEP, you should consider taking precautionary measures to avoid getting HIV moving forward, such as taking PrEP and using condoms. See The Basics of HIV Prevention for more tips to prevent HIV.
If you test positive for HIV after taking PEP, it’s best to talk to your health care provider about the next steps. Generally, if you test positive for HIV, you will do additional testing (lab tests) before getting prescribed an HIV treatment regimen. See Just Diagnosed: Next Steps After Testing Positive for HIV for more information.
This fact sheet is based on information from the following sources:
From CDC:
From MedlinePlus:
Also see the HIV Source collection of HIV links and resources.