The following adverse reactions have been reported with phenytoin.
Body as a Whole: Anaphylactoid reaction, and anaphylaxis.
Cardiovascular System: Hypotension has been observed.
Central Nervous System: Adverse reactions in this body system are common and are usually dose-related. Reactions include nystagmus, ataxia, slurred speech, decreased coordination, and mental confusion.
Dizziness, vertigo, insomnia, transient nervousness, motor twitching, headache, parasthesia and somnolence have also been observed. There have also been reports of phenytoin-induced dyskinesia, including chorea, dystonia, tremor, and asterixis, similar to those induced by phenothiazine and other neuroleptic drugs.
A predominantly sensory peripheral polyneuropathy has been observed in patients receiving long-term phenytoin therapy.
Connective Tissue System: Coarsening of the facial features, enlargement of the lips, gingival hyperplasia, hypertrichosis, and Peyronie's disease.
Gastrointestinal System: Acute hepatic failure, toxic hepatitis, liver damage, vomiting, nausea, constipation.
Hematopoietic System: Hematopoietic complications, some fatal, have occasionally been reported in association with administration of phenytoin. These have included thrombocytopenia, leukopenia, granulocytopenia, agranulocytosis, and pancytopenia with or without bone marrow suppression. Macrocytosis and megaloblastic anemia have also occurred. Lymphadenopathy including benign lymph node hyperplasia, pseudolymphoma, lymphoma, and Hodgkin's disease have been reported.
Immunologic: Hypersensitivity syndrome/Drug reaction with eosinophilia and systemic symptoms (HSS/DRESS), systemic lupus erythematosus, periarteritis nodosa, and immunoglobulin abnormalities.
Injection Site: Local irritation, inflammation, tenderness, necrosis, and sloughing of skin have been reported with or without extravasation of intravenous phenytoin. Edema, discolouration and pain distal to the site of injection (described as "purple glove syndrome") have also been reported.
Dermatologic System: Dermatological manifestations, sometimes accompanied by fever, have included scarlatiniform or morbiliform rashes. A morbiliform rash (measles-like) is the most common; other types of dermatitis are seen more rarely. Other more serious forms which may be fatal have included bullous, exfoliative, or purpuric dermatitis, lupus erythematosus, Steven-Johnson syndrome, and toxic epidermal necrolysis.
Special Senses: Taste perversion.