As nouns the difference between epi and pre
is that
epi is a syringe or autoinjector filled with epinephrine, most frequently used for the treatment of acute allergic reactions to avoid or treat the onset of anaphylactic shock or
epi can be (category theory) abbreviated form of epimorphism while
pre is (slang) precum, cowper's fluid, pre-ejaculate.
As a preposition pre is
before (something significant).
As a verb pre is
(slang) to precum, to pre-ejaculate.