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Hello. Thanks for the question.
It's not uncommon for dogs or cats to be wobbly and "drunk" after starting Phenobarbital. If they started him on oral Phenobarb, it can go on like this for 2-4 weeks before the body is used to the medication.
If it was just a one time injection, then the signs usually resolve within 2-3 days. You should be getting toward the end if he was not continued on oral Phenobarb at home.