CYP2D6 and invasive breast carcinoma: Finally, while there have been some studies demonstrating an association between CYP2D6 metabolism and/or endoxifen levels in patients with invasive cancer4–6,11, the one study evaluating CYP2D6 metabolism and outcomes in the NSABP P1/P2 clinical trials demonstrated no association between CYP2D6 metabolism and the risk of an invasive/non-invasive breast cancer in subjects taking tamoxifen 20 mg/day for primary prevention7.