Still is not recognized here in the US it's a toxic effect unrecognizable for lack of information provided to health care professionals here but is provided internationally here
http://www.inchem.org/documents/pims/pharm/pim416.htm2.2 Summary of clinical effects
Onset of symptoms including lateral nystagmus, ataxia,
and drowsiness occurs within 1 to 2 hours after ingestion and
may persist for about 4 to 5 days.
In more severe cases, horizontal nystagmus, coarse tremor and
inability to walk may be observed.
In very severe poisoning, conciousness is impaired but coma
is rarely observed.
2.3 Diagnosis
Diagnosis of phenytoin poisoning is clinical and based
on history of overdose and/or access to phenytoin and the
presence of specific clinical features especially nystagmus,
dysarthria and ataxia. The diagnosis may be confirmed in the
laboratory by measurement of an elevated serum phenytoin
level but the levels do not always correlate precisely with
the clinical severity of the intoxication.
Been there done that but still have to fight off Neurologist from giving her a loading dose for seizure caused from antibiotics most of the time all because they don't know any better, and who am I.
She was unable to walk for years until she stopped taking this.