What is PEP? Expand Post-Exposure Prophylaxis (PEP) are medicines taken after being potentially exposed to HIV to prevent infection. PEP must be taken within 72 hours (three days) and ideally should be taken within 24 hours. PEP is not a ‘morning after pill’ for HIV, and it’s not guaranteed to work. It’s meant as an emergency measure to be used as a last resort, such as if a condom fails during sex. Taking PEP will not protect you from other sexually transmitted infections or unwanted pregnancy.