Increased clearance of phenytoin as the presenting feature of infectious mononucleosis

A 19-year-old man with posttraumatic seizures under good control with phenytoin, 500 mg/day, presented with three seizures. His blood level of phenytoin, previously in a therapeutic range, was 4.2 μg/ml. No systemic or laboratory evidence of infectious mononucleosis was found. Ten days later, the blood picture became compatible with infectious mononucleosis, at which time his phenytoin clearance returned to normal.