This is an important message because millions of women globally are prescribed tamoxifen for the treatment or prevention of breast cancer.20,40 The proportion of poor and ultrarapid CYP2D6 metabolism has been estimated to be 5.4% and 3.1% in Europe,35 1.9% and 4.6% in the Americas, and 0.4% and 21.2% in Oceania, respectively.41 Therefore, the effect of poor and ultrarapid CYP2D6 metabolism on tamoxifen treatment is unlikely to be negligible; it might be associated with a less optimal therapy in hundreds of thousands of patients with breast cancer. Here, CYP2D6 is linked to breast cancer.