Post-Exposure Prophylaxis (PEP)

Post-exposure prophylaxis (PEP) is a technique used to prevent infection after someone has been exposed to a pathogen. It is used after known or suspected exposures to infectious agents.

Antiretroviral drugs on a table

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For example, a nurse who has been stuck by a needle thought to be contaminated with HIV might be given antiretroviral drugs to keep her from becoming infected. Similarly, a person who had a suspected exposure to anthrax would be given appropriate antibiotics.

Post-exposure prophylaxis is so named because it is given after (post) someone has been put at risk of an infection (exposure). Prophylaxis is another way of saying prevention. Post-exposure prophylaxis is generally only used to prevent infections that are considered to be highly dangerous.

Differences Between PEP and PrEP

It is important to distinguish post-exposure prophylaxis (PEP) from pre-exposure prophylaxis (PrEP) when talking about HIV. Post-exposure prophylaxis is generally provided as a short course of drugs that can be used to prevent HIV in groups where there has been a known risk event - such as a medical professional coming into contact with infected blood or a person being sexually assaulted by someone who may have been infected with the virus or had sex with someone with the virus with a broken condom. Pre-exposure prophylaxis, on the other hand, is being discussed as a long-term therapy to prevent infection in individuals who are at high-risk for HIV. It might, for example, be used by HIV-negative individuals who are in serodiscordant couples, where their partners are infected with the virus and there is a continuing risk of exposure.

Scientists and doctors have several concerns about the safety of pre-exposure prophylaxis. One of the biggest concerns is that people will take their medications unreliably and end up becoming infected with a drug-resistant strain of HIV that is substantially more difficult to treat. This is less of a concern with post-exposure prophylaxis for HIV. Unlike PrEP, PEP is only given for a short period of time (usually 4 weeks), and it is, therefore, easier for people to use correctly and consistently.

Fun fact: Condoms are also sometimes referred to as prophylactics, or "Pros" a term that accurately describes their ability to prevent both disease and pregnancy.

Alternate spellings: PEP, postexposure prophylaxis, post-exposure prophylaxis

Common misspellings: PrEP, Pre-exposure prophylaxis


Post-exposure prophylaxis to prevent HIV infection is thought to be highly effective. However, there is very little actual research on the topic, since failing to provide such prophylaxis to people who might need it would be considered to be highly unethical. What has been shown, in at least one case-control study, is that people who become infected after a known occupational exposure to HIV are much less likely to have been given prophylaxis. This suggests that the technique works, and there are also good biological reasons to assume that it would. It is just very difficult to demonstrate experimentally.

Despite the lack of hard evidence of its efficacy, post-exposure prophylaxis is widely accepted as the standard of care after a needle-stick or other occupational exposure to HIV. In addition, some researchers are studying the use of post-exposure prophylaxis after other types of exposures to the virus — such as through risky sex or injection drug use. Although there are some concerns that the use of PEP in such situations could increase risky behaviors, that has not generally been found to be the case. Several studies have found that the use of PEP by high-risk men does not impact their risk behavior in either direction.

Post-exposure prophylaxis for HIV and other STDs is usually part of the standard of care for victims of sexual assault.

By Elizabeth Boskey, PhD
Elizabeth Boskey, PhD, MPH, CHES, is a social worker, adjunct lecturer, and expert writer in the field of sexually transmitted diseases.