I work for a veterinarian group,and am a licensed vet. Tech. The dosage is 1mg/lb., so your dog would be recommended to take 3-25mg Benadryl every 12 hours.Max. 3 1/2 tablets every 12 hrs.
My pet had an allergic reaction to a vaccination and I took him to emergency. Benadyl can only be used in controlled situations like in your vet's office. If your vet agrees, you would leave your pet for a day, let the vet administer the meds and see how he does.