PEP stands for post-exposure prophylaxis. It involves taking antiretroviral medicines as soon as possible, but no more than 72 hours (3 days) after you may have been exposed to HIV, to try to reduce the chance of becoming HIV-positive. These medicines keep HIV from making copies of itself and spreading through your body. Two to three drugs are usually prescribed, and they must be taken for 28 days. PEP is not always effective; it does not guarantee that someone exposed to HIV will not become infected with HIV.
PrEP stands for pre-exposure prophylaxis. The word prophylaxis means to prevent or control the spread of an infection or disease. The goal of PrEP is to prevent HIV infection from taking hold if you are exposed to the virus. This is done by taking one pill every day. These are some of the same medicines used to keep the virus under control in people who are already living with HIV