I have a lot of experience w/ allergies, so I feel your pain!
Benadryl is usually not a good option for dog allergies, and is usually not used by Vet Derms bc of it's very short half-life; you end up having to give it 4x a day in order for it to even be therapeutic and evaluated properly. Temaril-P can be a nice option for seasonal allergies, but not great for year 'round issues (like dust or whatever) bc it has steroid.
How was a food allergy diagnosed? Food allergies can really only be diagnosed by doing an elimination diet first. Otherwise, it's just a guess -- and food allergies are actually very rare - accounting for only about 10% of allergies in dogs.
You should definitely read this thread:
http://www.yorkietalk.com/forums/die...tart-here.html