Primidone is slowly absorbed after oral administration, with peak levels occurring 2 to 4 hours after dosing.  Primidone is rapidly converted to PEMA and phenobarbital in dogs.  All three metabolites PEMA, primidone and phenobarbital have similar anticonvulsant actions, however primidone is metabolized very rapidly in dogs and contributes very little to the overall anti-seizure activity.  Phenobarbital, which is metabolized slowly accounts for about 90% of the overall anti-seizure benefit of the medication. 

Because Primidone is rapidly converted into phenobarbital in dogs, there is some question as to whether it has any advantages over using phenobarbital alone.  The disadvantage of primidone is that studies have shown dogs treated with primidone were more likely to show laboratory evidence of liver changes.   Based on these factors phenobarbital is generally chosen over primidone as an initial drug to control seizures.   However, there are a small percentage of dogs who will not respond to phenobarbital, but will respond to primidone.