A clinical interaction study of fulvestrant with midazolam (substrate of CYP 3A4) demonstrated that fulvestrant does not inhibit CYP 3A4.
Clinical interaction studies of fulvestrant with rifampicin (inducer of CYP 3A4) and ketoconazole (inhibitor of CYP 3A4) showed no clinically relevant change in fulvestrant clearance.
Dose adjustment is therefore not necessary in patients who are receiving fulvestrant and CYP 3A4 inhibitors or inducers concomitantly.