Will taking PrEP after unprotected sex prevent HIV? PrEP should not be used as a stand-alone dose after unprotected sex, it is designed to be taken before being exposed to the risk of HIV. Post-Exposure Prophylaxis (PEP) may be appropriate after a single high-risk event to prevent HIV. PEP must be started as soon as possible to be effective—and always within 72 hours of a possible exposure. For more Information click here.