Post-Exposure Prophylaxis (PEP) is an antiretroviral medication (ART) option offer to people AFTER being possibly exposed to HIV to reduce the risk of becoming infected.
PEP should only be used in emergency situation, and should be started within 72 hours after a recent potential exposure to HIV. Having said that, every hour counts, and the sooner you start on PEP the better.
PEP is not a substitute from HIV prevention methods, such as Pre-Exposure Prophylaxis (PrEP), using condoms effectively every time you have sex, using clean, new and sterile needles every time you inject.
Though PEP is effective, it is not 100% guarantee either. Hence, save sex and safe injection practices are advisable when you are on PEP. Theses extra measures are to prevent you from re-exposing to HIV again, and reducing the risks of transmitting HIV to others if you do become infected when you are on PEP.