Dexamethasone may lower the level of ethinyl estradiol in your blood. Rifampicin, phenytoin, primidone, rifabutin, griseofulvin, topiramate, modafinil, ritonavir, some protease inhibitors and barbiturates can do the same.
These medicines are often taken together with some care. Your doctor may change a dose, space them apart or check on you more often. Make sure your doctor and pharmacist know you take both.