Mephenytoin, (+)- is an (S)-enantiomer of anticonvulsant drug mephenytoin. Mephenytoin is usually administrated as a 1:1 racemic mixture of the (R)- and (S)-enantiomers. The marked stereoselectivity of 4’-hydroxylation of the phenyl ring of S-mephenytoin together with the relatively slow N-demethylation to R-PEH (R-5-phenyl-5-ethylhydantom) and even slower renal clearance results in a dramatic difference in the pharmacokinetic disposition of the S- and R-enantiomers of mephenytoin in man. As a consequence, S-mephenytoin provides a negligible contribution to circulating hydantoins, whereas R-mephenytoin is converted to the pharmacologically active demethylated product R-PEH which is the major circulating hydantoin during chronic administration of the racemic drug. Only the 4’-hydroxylation of the (S)-mephenytoin is absent in patents who are poor metabolizers of mephenytoin. It is not clear as to the clinical consequences of the accumulation of (S)-mephenytoin for the poor metabolizer phenotype. Indeed, it may be the extensive metabolizer who is at great risk of adverse effects by the formation of potentially toxic oxidative metabolites of (S)-mephenytoin.