5-Ethyl-3-methyl-5-phenylimidazolidine-2,4-dione, also known as Mephenytoin, is an active pharmaceutical ingredient (API) primarily used in the pharmaceutical industry as an anticonvulsant and antiepileptic agent. It is employed in the treatment of neurological disorders, particularly partial and generalized seizures, by modulating neuronal activity in the central nervous system (CNS). Historically, it has been utilized for managing epilepsy and other seizure-related conditions, though its use has declined due to potential adverse effects such as hypersensitivity reactions. Additionally, it serves as a reference standard or intermediate in organic synthesis for developing related hydantoin-based compounds with therapeutic applications in CNS disorders.