This article is from the Canine Epilepsy FAQ, by Alicia Wiersma-Aylward with numerous contributions by others.
In order for any drug therapy to be effective, the amount of drug
found in the body (serum concentration) must be consistently
monitored. No two animals may react to the same dose in the same way.
Farnbach reports a sixfold variation in the ratio between daily dosage
and serum concentration was demonstrated in a large population of
epileptic dogs. In 3 dogs given roughly the same dose of
phenobarbitol, one dog's condition did not change, the second dog
achieved seizure control, and the third dog experienced toxicosis. The
amount of drug found in the body correlates much better with seizure
control than daily dosage.
If your dog is on medication, work with your veterinarian in observing
your dog and testing his/her serum levels to ensure he/she is
receiving the appropriate amount of drug to achieve control and avoid