my parents had a saint bernard that was crazy afraid of thunder.
my dad was told by a neighbor to give him a benadryl capsule hidden in something like a piece of lunch meat or chunk of cheese. i would sit out there with him petting him & letting him lay his head in my lap until the benadryl knocked him out.
the benadryl never seemed to make the dog sick or anything and we never saw any major problems cuz he ONLY got it when it was needed.
i'm sure a vet would say "no" to it, but it worked on that dog....