Phenytoin-d5 (5,5-Diphenylhydantoin-d5) is deuterium labeled Phenytoin. Phenytoin (5,5-Diphenylhydantoin) is a potent Voltage-gated Na+ channels (VGSCs) blocker. Phenytoin has antiepileptic activity and reduces breast tumour growth and metastasis in mice[1][2].