Phenytoin-dexamethasone interaction: a previously unreported observation

A possible drug interaction between phenytoin and dexamethasone was evaluated by comparing serum phenytoin concentrations in 23 patients receiving both drugs with those of a control group of 15 patients receiving phenytoin only. Serum phenytoin concentrations obtained 24 hours after a loading dose of 11 mg per kilogram body weight administered intravenously and 13 mg/kg given intramuscularly were examined. The total amounts of dexamethasone administered during the 24-hour period ranged from 16 to 150 mg. The average serum phenytoin concentration was 17.28 +/- 3.49 micrograms/ml in patients receiving both drugs, as compared to 12.48 +/- 3.52 micrograms/ml in patients receiving only phenytoin (p less than 0.001). Our results suggest that serum phenytoin concentrations should be monitored in patients receiving concurrent dexamethasone therapy.