Pre-exposure prophylaxis (PrEP) is an HIV prevention strategy that uses antiretroviral drugs to protect HIV-negative people from HIV infection. People take antiretrovirals (ARVs) when they are at risk of exposure to HIV, in order to lower their risk of infection.

Research suggests that PrEP is highly efficacious in preventing the sexual transmission of HIV, as long as the drugs are taken regularly, as directed. However, PrEP does not prevent other sexually transmitted infections or pregnancy.

PrEP is one of several ways in which antiretroviral drugs can be used to prevent HIV transmission:

• ARVs given to an HIV-negative person before possible exposure to HIV: pre-exposure prophylaxis (PrEP).

• ARVs given to an HIV-negative person after possible exposure to HIV: post-exposure prophylaxis (PEP).

• ARVs taken by an HIV-positive woman during pregnancy and childbirth: prevention of mother-to-child transmission (PMCT).

• ARVs taken by an HIV-positive person: treatment as prevention (TasP).

The principle of PrEP is similar to that of antimalarial tablets used to prevent malaria when travelling in tropical countries.

Somebody who does not have HIV takes enough antiretrovirals (ARVs) for there to be high levels of the drugs in their bloodstream, genital tract and rectum before any exposure to HIV. If exposure occurs, the ARVs stop the virus from entering cells and replicating. This prevents HIV from establishing itself and the person remains HIV negative.

The antiretrovirals which are currently used as PrEP (Truvada, a pill containing two drugs, tenofovir and emtricitabine) were chosen because they have limited side-effects, have few problems with drug resistance, reach high levels in the genital tract and rectum, and remain in the body for a relatively long time.

Although the term PrEP is often used to refer to ARVs in oral tablets, other delivery methods are possible. A vaginal ring, or a microbicide gel that is placed in the vagina or rectum, can provide ARVs – the approach is sometimes called ‘topical’ PrEP. However, so far studies of microbicide gels for women have not been successful, largely due to social barriers and poor adherence.