Can a dog take Benedryl if they have glaucoma?

For the most part, no. Dogs with Allergies tend to have skin and gastrointestinal tract issues, not the upper respiratory signs shown by humans. Therefore, giving a dog Benadryl to reduce allergy-related inflammation is probably not going to do anything to resolve an eye issue. However, a veterinarian who has examined your dog and determined the cause of the eye issue may prescribe Benadryl for your dog.