Tamoxifen is a hormone therapy used in the treatment and prevention of hormone-dependent breast cancers. Estrogen dependent breast cancers are malignant tumors that express receptors for the estrogen. Tamoxifen acts as selective estrogen modulators with antiestrogen effect on breast tissue, which prevents the proliferative effect of estrogen on cells and consequently the growth of tumors. In order to achieve a therapeutic effect, tamoxifen in the body must first be metabolized mainly by the CYP2D6 enzyme to its main active metabolite, endoxifen. The gene that codes for the CYP2D6 enzyme is highly polymorphic, prone to variability, which causes phenotypically diverse enzymatic activity. By translating the genotype into the phenotype, four CYP2D6 phenotypes were defined: very fast, normal, intermediate and slow metabolizers. This affects the effectiveness of drugs that are metabolized by CYP2D6. With the development of personalized medicine and an individual approach to treatment, guidelines were developed for the selection and dosage of some drugs according to the individual's genetic profile. Thus, very fast and normal metabolizers of tamoxifen achieve a therapeutic effect using standard doses. In contrast, slow and intermediate metabolizers have a lower concentration of endoxifen in plasma, which reduces the therapeutic effect of tamoxifen and increases the risk of disease relapse, so alternative therapy is recommended for them.