I have allergies. when i come into contact with allergen, should i use epipen (epinephrine) before symptoms progress even though i'm just itchy? or should i wait?

Depends: If we are talking about severe food allergies like to peanut then I say shoot first ask questions later even before symptom onset. Unless you have a history of significant cardiovascular conditions then the risk vs benefits is in favor of taking the shot. Deaths from peanut allergic reaction has been linked to delays in using epinephrine.

EpiPen (epinephrine) after: EpiPen (epinephrine) is not intended for prophylactic use since its action is rather transient. It should be used however at the earliest phase of an anaphylactic reaction when you have shortness of breath, fainting, vomiting etc.

Depending on what: the allergen is and what reaction you have had in the past....also depends on where the itching is.For example, if you have itching in your throat or around your mouth and have had an allergic reaction needing epi in the past,you should not wait.You should use the epipen (epinephrine) and get to a hospital.This is more likely to be the case with a food allergen or bee sting.Please also discuss with your doctor.