I would not use Benadryl - it does not work well for allergies in dogs. Also, it has a very short half life - so, to remain therapeutic, you have to give it 4x a day - a total pain!
Vet Derms recommend either Claritin or Zyrtec as these are both 24 hr meds (once a day).
But *most* allergy dogs need more than that, and require prescription meds -- read this thread for more about allergies and med options:
http://www.yorkietalk.com/forums/die...tart-here.html