Abstract
Different strategies have been proposed to individualize tamoxifen treatment in order to improve recurrence-free survival in estrogen receptor (ER)-positive breast cancer. To date, the debate remains on which strategy should be used. The objective of this viewpoint is to highlight Therapeutic Drug Monitoring of endoxifen, the active tamoxifen metabolite, as the preferred methodology compared to CYP2D6 genotyping for individualizing tamoxifen therapy for ER-positive breast cancer patients treated in the adjuvant setting.