Phenytoin hypersensitivity hepatitis and mononucleosis syndrome.

We present a patient who developed a mononucleosis syndrome 6 weeks after starting phenytoin therapy for a seizure disorder. Improvement followed initial discontinuation of the phenytoin, but near-fatal hepatic necrosis resulted from intravenous phenytoin rechallenge. A serious reaction with a 18% overall mortality, phenytoin hepatitis may closely resemble infectious mononucleosis with fever, pharyngitis, lymphadenopathy, and atypical lymphocytosis. Continued administration of phenytoin or later rechallenge may increase the severity of the reaction.