Post-exposure prophylaxis (PEP) is a course of anti-HIV medication that may block HIV infection. Although no antiretroviral has been licensed for use as prophylaxis against HIV infection, some hospitals offer PEP to people after a possible exposure to HIV.

PEP only works if treatment is started soon enough after exposure and lasts long enough, usually around four weeks. Therefore, it should not be considered as a ‘morning after pill’ for HIV, as a full course of PEP requires consistent adherence over many weeks, often in the face of side-effects, at a time that may be stressful.

Treatment does not need to continue indefinitely because the aim of treatment is to prevent HIV from entering cells in the body: infection will either become established or will be aborted within that period. It can take HIV between one and five days to become established in the CD4 T-cells and lymph nodes after exposure^[1][2]. It is thought that PEP acts during this time to prevent the virus taking hold, and therefore to prevent the exposed person becoming HIV-positive.

Although some animal studies have shown that PEP can prevent infection with HIV or similar viruses, studies in humans are often much harder to interpret, as they rely on observations of the number of infections in groups of people who may not otherwise have become infected with HIV. Nevertheless, a number of studies have shown that PEP is promising treatment for the prevention of infection in people who may have been exposed to the virus, either through exposure at work or through sex, including sexual assault and accidents.