– Phenytoin-d10 is intended for use as an internal standard for the quantification of phenytoin by GC- or LC-MS. Phenytoin is an anticonvulsant agent and active metabolite of fosphenytoin. Phenytoin is formed from fosphenytoin by tissue phosphatases. It inhibits neuronal voltage-gated sodium channels in a voltage-dependent manner. Phenytoin reduces the neuronal firing frequency and decreases the amplitude of excitatory post-synaptic potentials (EPSPs) in electrically stimulated rat corticostriatal slices. It protects against seizures induced by maximal electroshock (MES) in mice. Formulations containing phenytoin have been used in the treatment of tonic-clonic seizures and status epilepticus.