I've had a allergic reaction. there were hives everywhere. i've been taking meds and recently the hives turned into dark marks that look like scars.

See a dermatologist: Cause of hives? Not always allergy. If it was allergy, then allergy to what? Benadryl is a brand name for diphenhydramine which is useful mainly as a sleeping pill. There are much better antihistamines which will be less likely to make you sleepy. Do not drive after taking Benadryl. If a rash, not hives, no antihistamine will help. Start with a precise diagnosis if possible.