Low Dose TamOxifen and LifestylE Changes for bReast cANcer prevenTion (TOLERANT)

Inclusion Criteria: Women between 18 and 70 years old; Healthy participants carriers of a germline pathogenic/likely pathogenetic variant (BRCA1 or BRCA2 or PALB2), or with a > 5% risk of developing breast cancer at 10 years according to the "Tyrer-Cuzick model", or with a previous diagnosis of intraepithelial neoplasia of the breast (atypical ductal hyperplasia (ADH), lobular carcinoma in situ (LCIS), estrogen receptor (ER) positive ductal carcinoma in situ (DCIS)) within the last 3 years; Ability to understand and the willingness to sign a written informed consent document; Eastern Cooperative Oncology Group (ECOG) Performance Status ≤1; 5a. For high-risk strata: A negative mammogram or any radiological image based on age and center protocol screening within 6 months before baseline visit; 5b. For IEN Strata: A negative mammogram within 12 months before baseline visit 6. A negative transvaginal ultrasound within 6 months before baseline visit. Exclusion Criteria: Diagnosis of ER-negative (<10%) DCIS, or history of invasive breast cancer; Previous treatment with selective estrogen receptor modulators (SERMs) or any other hormonal treatment for breast neoplasms; BMI < 18.5 Kg/m2 and/or Malnutrition Universal Screening Tool (MUST) score ≥2 and/or any current or past eating disorders; Any diagnosis of invasive neoplasia, except non-melanoma skin cancer, in the previous 5 years; Any tamoxifen contraindications (abnormal liver function, previous ischemic heart disease, endometrial disorder, previous deep venous thrombosis, history of pulmonary embolus, current or suspected glaucoma, retinopathy, and cataract); Current use of warfarin or other anticoagulant drugs Bilateral mastectomy; Pregnancy or desire to become pregnant in the subsequent 9 months after treatment cessation; Diabetes or any other clinical condition that, at the investigator's discretion, contraindicates the proposed intervention. No hormonal contraception is allowed during study intervention. Non-hormonal methods will be advised for women of childbearing potential (WOCBP).