You need to talk with your veterinarian about this - it is possible some formulations of Benadryl could interact with your dog's thyroid medication and cause major problems.
You can give 0.5-1mg of benadryl per pound
no you shouldn't wake a sleep walker it can cause seisures or tempermental disorders
Benadryl is for allergies. Don't think it'll help with the kind of poisoning chocolate can cause in dogs.
Yes, a dog can overdose on Benadryl, and it doesn't take as much as you might think.
While Benadryl or diphenhydramine can be used safely for dogs, if proper dosage is carefully applied, diphenhydramine is not an anticonvulsant, and as such will likely not work for this application. In short, not deadly; not effective either.
Marijuana
Yes, a dog can overdose on Benadryl, and it doesn't take as much as you might think.
This will depend upon the size of the dog and the exact formulation of Benadryl you are asking about.
What does your dog's Vet say?
No, this drug is for allergies.
No, this would be a very bad idea. First, dogs need a much different dose of Benadryl than humans do, and skin absorption of Benadryl is very different between humans and dogs. You could cause a bad overdose with this without meaning to. Second, Benadryl cream is formulated to be placed on the skin ONLY - dogs tend to lick anyplace that is irritating them and are likely to lick this stuff off and eat it, potentially causing a bad medical reaction to the ingredients in the cream. Third, dogs really don't need a topical Benadryl - veterinarians use it to deal with bee stings and mast cell tumors, but those are both pretty rare and not something you would be handling on your own.