4-Hydroxymephenytoin is a metabolism of mephenytoin, which is an antiepileptic drug, and used as a CYP2C19 substrate. 4-Hydroxymephenytoin (4'-Hydroxymephenytoin) is used for developing UltraPerformance LC (UPLC) tandem mass spectrometry assays for evaluating CYP probe drugs and their relevant metabolites in human urine or plasma samples.