Is it safe to give a large dog a benadryl?

[xmelax35]

Is it safe to give a large dog a benadryl? We gave him a flee bath and also put flee medicine on him, but he still will not stop the scratching. He has done this for the past few years at this same time of year (scratches himself bald). He is a chocolate lab. The vet said to give him an oatmeal bath and sold us the flee medication, but it just does not work. They won't answer if it is okay to give him benadryl when I called. They just said we could make Another appointment. (we just had one recently - when we were given the high priced flee medicine to put on the dogs). Someone told me they have a lab and once in awhile when the dog goes through that (allergic to the flees they said) they give their dog a benadryl (not all the time just if they dog is at it all night and does not get any sleep) to help with the itching.

Serious answers only please and thank you ahead of time.