If your dog has not had blood work done to evaluate organ function recently, that should be done. Since she has been on this medication for 12 years, that must mean that she is an older pet and we do not want to miss something that may be going on internally and simply assume it is the same old seizures that she has had in the past.
The most common anti-seizure medication used in dogs is phenobarbital. In many dogs, this will control the seizures very well. In some dogs that do not respond to phenobarbital, potassium bromide is added. Some vets will start out with potassium bromide rather than phenobarbital - and then add the phenobarbital if potassium bromide is not adequate. Either way is okay. Blood tests to determine the level of both of these medications should be done, especially if there are break through seizures, to make sure they are being absorbed properly and are at a therapeutic level within the blood stream.
There are still some dogs will continue to have seizures while on both of these medications, and there are a handful of drugs that can be tried in those dogs to see if they will respond to those. These drugs include gabapentin, Keppra, or Zonisamide.
Dilantin used to be used for seizure control in dogs but has lost favor because it does stay in the blood system for that long so it is less effective than the phenobarb or potassium bromide. Twelve years ago it was very appropriate to use, but it just isn't used much any more. While it is not necessarily a bad choice, it is just that we have better options now.
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