(R)-5-Ethyl-3-methyl-5-phenylimidazolidine-2,4-dione, also known as the (R)-enantiomer of mephenytoin, is used as an active pharmaceutical ingredient (API) in the treatment of epilepsy. As a hydantoin derivative, it acts as an anticonvulsant by modulating sodium channels to stabilize neuronal membranes and reduce seizure activity. It is particularly indicated for partial seizures and secondarily generalized tonic-clonic seizures, with historical use in refractory cases including some absence seizures. Due to potential side effects such as hematologic toxicity, its use has declined in favor of safer alternatives. It is typically administered orally and requires monitoring of plasma levels for therapeutic efficacy.