Is it Phenobarbitol? I'm so sorry! I had a very sever epileptic dog I just lost a little over a month ago, and sadly, had a huge range of drugs we had to use to control her seizures. If he reacts so poorly and he's on Phenobarb, I'd ask about using Gabapentin for 2 reasons. 1: It's becoming more common as a anti-epilepsy drug in dogs, and in my experience and that of clients in the clinics I've worked in, it seems to be better tolerated then some of the other epilepsy drugs. 2: For a dog that age, there's probably some arthritis and age-related pain going on. Gabapentin may also help with that.
Sometimes it's a matter of waiting it out and side effects will lesson with time - But that said, if it's that bad, it never hurts to ask about trying a different drug and seeing if that works better. Your vet may say no - But never hurts to discuss it.
__________________ Momma to King Louie 
Missing my angels  California Lily and  Benjamin Button, forever in my heart |