PEP means "post-exposure prophylaxis"; PrEP means "pre-exposure prophylaxis". You take PrEP regularly (if you know you're at risk due to sexual habits etc whate

PEP means "post-exposure prophylaxis"; PrEP means "pre-exposure prophylaxis". You take PrEP regularly (if you know you're at risk due to sexual habits etc whatever), but only take PEP in response to a specific possible-exposure event (and not already on PrEP). You want pre-exposure instead of post-exposure because post-exposure is very hard on your body and makes you feel sick.