PEP stands for Post-Exposure Prophylaxis and is a short course of HIV medicines taken very soon after a possible exposure to HIV in order to prevent the virus from taking hold in your body. [1] PEP must be started within 72 hours of possible exposure to HIV, and every hour counts. [2] PEP should be used only in emergency situations. It is not meant for regular use by people who may be exposed to HIV frequently. [3] PEP is effective in preventing HIV infection when taken correctly, but it's not 100% effective. Side effects can include nausea, and you will need to return to your health care provider at certain times while taking PEP and after you finish taking it for HIV testing and other tests.