Usually, Benadryl does not work well at all for allergy relief in dogs, but you could try. It has a very short half-life, so you have to give it 4x a day for it to be continuously effective and therapeutic. Better OTC options for dogs are Zyrtec or Claritin, which are 24 hour pills.
But there are prescription options which generally work much, much better in dogs. Read this thread for how to approach allergies:
http://www.yorkietalk.com/forums/die...tart-here.html