Inclusion Criteria:

          1. Premenopausal women aged 18-50 years;

          2. ECOG PS: 0-2 points;

          3. Invasive breast cancer confirmed by histology with ER ≥ 10% (all test results should
             be reviewed and confirmed by Department of Pathology of the participant institution);

          4. Participants have completed the standard local radical treatment (modified or
             conservative radical mastectomy) with or without neo-adjuvant/adjuvant chemotherapy or
             radiotherapy;

          5. Participants must be able to understand this study and are willing to participate,
             agree to genotype screening and sign informed consent form with good compliance and
             cooperation in follow-ups;

          6. Polymorphism analysis showed that patients are CYP2D6 * 4, * 5, * 10, * 14, * 17, * 41
             allele carriers;

          7. Hemoglobin ≥ 90g/L, neutrophils ≥ 1.5 × 109/L, platelets ≥ 75 × 109/L, AST and ALT ≤
             2.5 times the upper limit of normal (ULN), serum creatinine and urea nitrogen ≤ ULN.

        Exclusion Criteria:

          1. Patients have previously received neoadjuvant endocrine therapy or have started
             adjuvant endocrine therapy;

          2. There are any comorbidities that may increase the level of sex hormones: such as
             pituitary adenomas, ovarian tumors, thymic carcinomas, etc.;

          3. There are any comorbidities that may reduce the level of sex hormones such as
             hyperthyroidism, hypothyroidism, cirrhosis, severe malnutrition, Turner syndrome, lack
             of sex hormone synthetase, intracranial tumors, pituitary atrophy etc.;

          4. Patients have undergone or planned to conduct ovariectomy or ovarian function
             inhibition;

          5. Patients needs to take other medicines which can influence the activity of CYP2D6
             (such as fluoxetine, paroxetine, quinidine, bupropion), CYP3A4 (such as erythromycin,
             acetylspiramycin, ritonavir, ketoconazole, nicardipine);

          6. Patients have been treated with other trial medications in the past 2 weeks;

          7. Pregnant or lactating women (women of childbearing age must have a negative pregnancy
             test within 14 days of the first dosing, and if pregnant, Patients are required for
             ultrasound examination to exclude pregnancy);

          8. Women of childbearing age who are not willing to take effective contraception during
             treatment;

          9. There are serious non-malignant tumor comorbidities that may affect long-term
             follow-up;

         10. Patients have family history of endometrial, ovarian or other gynecologic
             malignancies;

         11. Transvaginal ultrasound suggested more serious ovarian abnormalities or endometrial
             thickening;

         12. Patients have had thrombotic events such as cerebrovascular accident (including
             transient ischemic attack), deep venous thrombosis, and pulmonary embolism within 6
             months prior to study initiation;

         13. Serious liver insufficiency with Child-Pugh C grade;

         14. Serious cardiac insufficiency with New York Heart Association (NYHA) grade ≥III;

         15. Patients are known severely allergic to study drug;

         16. Patients have history of other malignancies in the past five years, except for
             cutaneous basal cell carcinoma and cervical carcinoma in situ which have been cured;

         17. In other cases, the researchers don't think the subjects are suitable for participate
             in the study.