I'm so sorry I have no experience with meds and dosages, or what types of medications can be mixed with each other, so wouldn't want to give you any advice. I usually always ask my pharmacist for advice of this nature, they have much more knowledge than even doctors do regarding medications, dosages, and possible interactions or reactions to medications. I would think it would be the same reactions for humans and dogs, but you could call a pharmacist and get advice on if its okay to take a Benadryl at the same time as taking phenobarbital? It's been 4 hours since the phenobarbital, so it's probably worn off by now, and if he's not getting another dose, then it should be okay to give the Benadryl, again a pharmacist or your vet can give you advice. Hope Loki feels better soon.