Post Exposure Prophylaxis, or PEP, is a brief course of HIV medications taken as soon as possible after a potential HIV exposure to stop the virus from taking root in your body. It must be started within 72 hours (3 days) of a potential HIV exposure for it to be effective. Every hour matters. PEP should only be used in dire circumstances. It is not intended for regular usage by anyone who may often come into contact with HIV.