Interaction of dexamethasone with phenytoin.

Phenytoin and dexamethasone are frequently administered concurrently to prevent consequences of central nervous system pathology. Limited and conflicting data are available regarding an interaction between the agents. A patient receiving dexamethasone required a daily phenytoin dose of greater than 10 mg/kg to maintain the phenytoin serum concentration within the usual therapeutic range. The concentration increased by nearly 300% after dexamethasone was discontinued.