The (R) enantiomer of mephenytoin; can be demethylated by CYP2C9 to form the active metabolite 5-ethyl-5-phenylhydantoin (nirvanol); the ratio of (S)- to (R)-mephenytoin in urine following administration of the racemic mixture has been used to detect polymorphisms in drug metabolism by CYP2C19, as only (S)-mephenytoin is a substrate of CYP2C19