Gefitinib dose should go up to 500 mg daily if you are taking a strong CYP3A4 inducer such as rifampicin, phenytoin or a tricyclic antidepressant. Go back to 250 mg 7 days after the inducer is stopped.
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.