Increased serum phenytoin concentration following influenza vaccination

The effect of influenza vaccination on serum phenytoin concentration was studied in a group of long-term care patients with epilepsy. The subjects in this study were 15 men and 1 woman with a mean age of 76.7 years. All were white, receiving long-term phenytoin therapy for seizure disorders, on stable antiepileptic regimens for a minimum of eight weeks, and scheduled to receive an influenza vaccination. Baseline phenytoin concentrations were determined one or two days before vaccination. Subjects were vaccinated subcutaneously with 0.5 ml of an inactivated whole-virion trivalent influenza vaccine. Phenytoin therapy was maintained, and serum concentrations were determined on days 7 and 14 following the vaccination. Assays were performed with a standard enzyme immunoassay technique. No significant increases in mean serum phenytoin concentrations were observed on days 7 or 14 following vaccination. However, temporary increases of 46-170% occurred in four subjects, and in two of these subjects, the increases in serum phenytoin concentration could best be attributed to the influenza vaccination. Serum phenytoin concentrations may be temporarily increased in some individuals after influenza vaccination.