34yrs. Cerebral palsy . Epilepsy . 24hr care required. Been on tegretol cr and phenobarbitone for years to help control seizures. Neurologist prescribed keppra for 2yrs. No side effects but experienced 1-3 seizures daily. Weened off keppra and started sodium valproate . Within the first week she had experienced sedation, vomitting. Then bloods showed valpro levels low, dose increased. She experienced hypothermia , sedation, vomitting. She lost the ability to swallow food and drink. Bloods showed her phenobarb levels were toxic. Is this common or unheard of for these medications to interacte this way?