Valproate sodium is a widely applied agent in the treatment of epilepsy. Although Valproate sodium is effective in clinic, it is able to induce several side effects, including weight gain, thinned hair, loss of appetite, nausea, vomiting, hepatotoxicity, hematotoxicity, thrill, etc. However, the remarkable variability of the reactions to the drug -- the incidence of side effect or the outcome of the treatment -- has been observed among patients. Valproate sodium is metabolized by some enzymes in the liver to transform it into several unreactive chemicals for excretion. Among them there are two toxic metabolites catalyzed by the specific metabolic enzymes. This study is designed to explore the genetic variation among individuals in the key processes of the deactivation and elimination of Valproate sodium in order to find out whether these genetic factors are associated to the side effects or efficacy. The further understanding into the factors concerning on the drug might imply possible solution to minimize the incidence of side effects in epileptic patients.