PEP stands for post-exposure prophylaxis, and generally refers to the use of a drug to prevent an infection after exposure. It is used in a number of infections, including HIV. For HIV PEP, someone with an exposure to HIV (e.g. through a sexual encounter or a needlestick injury) can take 28-days of HIV medications to prevent infection. In the DISCO study, STI PEP means taking two doxycycline pills within the three days after sexual activity.