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Many adverse CNS reactions can occur during phenytoin therapy including dizziness, drowsiness, nystagmus, ataxia, lethargy, coma, seizures, choreoathetosis, and dystonic reaction. The severity
of these adverse reactions increases as serum concentrations
of phenytoin increase. While lethargy, dizziness, and drowsiness may occur at therapeutic serum concentrations, ataxia, coma, and drug-induced seizures usually occur at supra-therapeutic concentrations.