My Cats Response To Phenobarbatol Makes Him Drowzy And Unable

my cats response to phenobarbatol makes him drowzy and unable to walk properly, hes 21 yrs old. Can I withdraw the medication completely and if so what is the likely hood his seizures will return. Is there a dose that can be given which balances effective control of seizures with minimum side effects Right now he is on one tablet daily and the drowziness causes him to fall over constantly. My vet says I can halve the dose but I wonder whether it is possible to stop it completely. What is the minimum dose you recommend to maintain effective control and minimum side effects Thanks Steve