He W, Grassmann F, Eriksson M, et al. - Via performing a prospective-retrospective study combining data from a clinical breast cancer register, the Swedish Prescribed Drug Register, and self-reported questionnaires, researchers sought to investigate the association between CYP2D6 genotype, discontinuation of tamoxifen therapy, and prognosis for breast cancer. CYP2D6 was genotyped in 1,309 patients with breast cancer who were treated with tamoxifen and were diagnosed from 2005 to 2012. Categorization of patients was done as follows: poor, intermediate, normal, or ultrarapid CYP2D6 metabolizers. Among poor, intermediate, normal, and ultrarapid CYP2D6 metabolizers, the 6-month discontinuation rates of tamoxifen were 7.1%, 7.6%, 6.7%, and 18.8%, respectively. The analysis revealed a worse prognosis for breast cancer for both poor and ultrarapid CYP2D6 metabolizers of tamoxifen compared with normal metabolizers after receiving a standard dose of tamoxifen. This U-shaped association seemed supporting the individualization of tamoxifen dosage.