Effect of influenza vaccine on serum anticonvulsant concentrations

Serum concentrations of the anticonvulsants phenytoin, phenobarbital, and carbamazepine were compared before and after patients received influenza vaccine. Serum drug concentrations were measured in patients at a state school for the mentally retarded who were receiving continuous anticonvulsant therapy with only one of the study drugs and taking no other medication regularly. Patients with hepatic or renal disease or other medical problems were excluded. All study patients had steady-state serum concentrations of the anticonvulsants before they were vaccinated with Influenza Virus Vaccine, USP, Types A and B, Whole Virus. Trough serum concentrations of anticonvulsants were measured by enzyme-mediated immunoassay technique immediately before vaccination and on days 7, 14, and 28. Data were excluded for patients who required dosage adjustments because of toxicity or seizures. On day 7, mean serum concentrations of phenytoin (15.16 +/- 5.52 micrograms/mL, n = 8) and phenobarbital (17.25 +/- 6.77 micrograms/mL, n = 27) were significantly higher than at baseline. Mean carbamazepine concentrations on day 7 (6.89 +/- 2.18 micrograms/mL, n = 20) were not significantly greater than baseline; however, there was a significant increase from day 7 to day 14. In patients who are receiving phenytoin, phenobarbital, or other drugs metabolized by the cytochrome P-450 system, serum concentrations of these drugs may increase as a result of influenza vaccination, and dosage adjustments may be necessary.