the idea of decreasing the Phenobarbital dosage over time. Avoid Advantage while decreasing the Phenobarbital.

Collies have a genetic defect in a gene that encodes for the cell membrane P-glycoprotein pump. This pump protects the brain from a build up of such drugs as Ivermectin, and Immodium A/D. When defective the drugs build up in the brain to toxic levels. Even part-Collies can be affected by this genetic defect.

Collies and drugs

Thus it's possible the active ingredient in Advantage could have caused the seizure. If you are able to wean the dog off of Phenobarbital without any more seizures occuring then that would be suggestive the Advantage caused the problem.

However, if the seizures start to return in the absence of Phenobarbital, then the bad news for you is that both the German Shepards and Collies have an increased incidence of epilepsy in their genetics.

Unfortunately even mixed breeds are not safe from the genetic defaults if both parents were carriers and/or had the defect phenotype.