PEP is highly effective to prevent you from getting HIV if taken within 72 hours after possible exposure to HIV. Once you started taking PEP, you will need to take it daily for 28 days. While taking PEP, it is important to use another HIV prevention method like using condoms while having sexual intercourse because PEP is not 100% effective to prevent HIV. Just to keep in mind, PEP is highly effective but not 100% effective.