The therapeutic effects of
dexamethasone,
methylprednisolone,
prednisolone,
prednisone (and probably other glucocorticoids) can be markedly reduced by
phenytoin. Fludrocortisone may similarly be affected. One study suggested that
dexamethasone may increase
serum phenytoin levels, but another study and two case reports suggest that an important decrease can occur. The results of the dexametha- sone adrenal suppression test may prove to be unreliable in those taking
phenytoin. Fosphenytoin, a prodrug of
phenytoin, may interact similarly with these corticoster- oids.
The significance of the falls in corticosteroid levels will depend on the disease the corticosteroid is prescribed for. Monitor the outcome of concurrent use closely, especially in transplant patients, and increase the dose accordingly. Consider monitoring
phenytoin levels if
dexamethasone is given.