After only one seizure I cannot fathom any vet useing medication at this point. Most vets want to witness a seizure first too ...sometimes a drop in glucose can cause one.
If a dog has a seizure and the owners rush to the vet, bloodwork will show something has happened or something "is" happening. If the owner waits even 12 hours...the bloodwork could be..98% of the time, normal.
A dog with epilepsy will have normal blood levels too. If they seizure only twice a year, no medication is needed. If they seizure more than twice a month or more frequent than they can still be controlled with medication. Usually as age sets in so will the episodes.
Phenobarbital is not a drug to fear, properly regulated doses can mean a full and happy life.
Many things can cause seizure's later in life...for instance a dog that suffered from canine distemper as a puppy and recovered. Treatments can vary.
Be The Kind Of Person Your Dog Thinks You Are.