My Lady is epileptic and has been on Phenobarbital for almost eight years. I have never heard of a vet prescribing it to calm dogs down before. Pheno is really hard on the liver which is why they hold off prescribing it for seizures until they absolutely have to. It's not even approved by the FDA for use in dogs.
Here is some information on PB:
http://www.canine-epilepsy-guardian-...nobarbital.htm
Are you sure your vet didn't suggest Benadryl? I've heard of that being used to calm a dog before grooming or a car trip.