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A clinical interaction study with midazolam (substrate of CYP 3A4) demonstrated that fulvestrant does not inhibit CYP 3A4. Clinical interaction studies 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.
Due to the structural similarity of fulvestrant and oestradiol, fulvestrant may interfere with antibody-based oestradiol assays and may result in falsely increased levels of oestradiol.
