First, what do you mean by "skin allergies"...? Most allergies in dogs are environmental (pollens, dust, grass, mites, etc...) and then some, rarely, to food. The symptoms of allergies often come out in the skin and also result in paw licking and chewing, and itching.
Benadryl is really rarely used in alleriges bc it has such a short 1/2 life and has to be given every 4-6 hours to be therapeutic...it doesn't usually work well in dogs for that. Vet Derms will often try Claritin or Zyrtec as OTC meds for dogs.
For other meds and more on allergies, def read this thread:
http://www.yorkietalk.com/forums/die...tart-here.html