Can excessive diphenhydramine actually make you itchy?

Yes: I have had several patients who had become allergic to Diphenhydramine after using it on the skin for poison ivy or other ailments, so yes, although rare, any previous use of Diphenhydramine can cause an allergic reaction to it, such as itching at first, then rash. This is a concern and must be documented because it is one of our go-to medicines for allergies.