HIV Prevention

Post-Exposure Prophylaxis (PEP)

Last Reviewed: July 10, 2026

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 to be effective. 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) and condoms.

 

What is post-exposure prophylaxis?

Post-exposure prophylaxis (PEP) refers to the use of HIV medicines to prevent HIV infection shortly after a possible exposure. PEP must be started within 72 hours (three days) after possible exposure to work effectively.

Although PEP can help prevent HIV transmission, it should be used in emergencies only 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 from PEP because it is taken regularly before someone is exposed to HIV. PrEP can be taken as a daily pill, or as an injection less frequently. For more information, see the HIVinfo fact sheet on Pre-Exposure Prophylaxis (PrEP).

Who should consider taking PEP?

PEP can be given to people who do not have HIV or who do not know their HIV status and who may have been exposed in the last 72 hours. This includes people who—

  • May have been exposed to HIV during sex
  • Shared needles or other equipment to inject drugs
  • Were sexually assaulted
  • May have been exposed to HIV at work (an occupational transmission)
  • Had any other suspected exposure (such as a needlestick injury)

If you think you were recently exposed to HIV, talk to your health care provider or an emergency room doctor about PEP immediately. The sooner PEP is started after possible HIV exposure, the better. Every hour counts.

What HIV medicines are used for PEP?

CDC provides guidelines on recommended HIV medicines for PEP, which include recommendations for specific groups of people, such as pregnant women or people with kidney problems.

The HIV medicines used for PEP are also commonly used for HIV treatment. When PEP is started soon after exposure, it prevents the virus from replicating or creating a latent HIV reservoir. Learn more in the HIVinfo fact sheets, The Stages of HIV Infection and What is a Latent HIV Reservoir?.

A health care provider will work with you to determine which PEP medicines are the most likely to be safe and effective for you.

How well does PEP work?

CDC’s preferred PEP regimens are most effective when started right away and taken every day for 28 days. In five studies using bictegravir- or dolutegravir-based PEP, no participants acquired HIV.

Although PEP can be very effective, its success depends on taking all medicines exactly as prescribed, avoiding new exposures while on PEP, and using other prevention methods during and after finishing the 28-day course. If you may be exposed to HIV again after completing PEP—such as if you have a partner with HIV—your health care provider may recommend starting PrEP to support long-term prevention.

While taking PEP, it is important to keep using other HIV prevention methods, such as using condoms correctly every time you have sex, and using new, sterile drug injection equipment only. PEP does not protect against other sexually transmitted infections (STIs), so condoms can help reduce the risk of HIV and other STIs. 

Does PEP cause side effects?

The HIV medicines used for PEP are well-tolerated but may cause side effects in some people. The most common side effects are generally mild and may include diarrhea, nausea, tiredness, and headache. Mild side effects can usually be treated or go away 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 caused by PEP medicines. Contact your health care provider immediately if you have side effects that make it hard for you to keep taking your PEP medicines.

What should you do after taking PEP?

After completing a full course of PEP, you should get tested to confirm whether the medicines successfully prevented possible HIV transmission.

If you test negative for HIV after finishing PEP, you should continue using HIV prevention methods, such as taking PrEP (if needed) and using condoms. See The Basics of HIV Prevention for more tips to prevent HIV.

Although unlikely, if you test positive for HIV after finishing PEP, it’s best to talk to your health care provider about the next steps. Generally, 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.