4cfc52b1__for_Professionals___Drugs_com__Interaction
[Drug or Test] Test, dexamethasone [Interaction] 
Phenytoin may cause slight decreases in urinary 17-hydroxycorticosteroids and 17-ketosteroids while urinary 6-β-hydroxycortisol excretion is increasedb Phenytoin may produce lower than normal values for the dexamethasone test
Phenytoin Sodium Monograph for Professionals - Drugs.com
Interaction