Probiotics for Abemaciclib-Induced Abdominal Discomfort.

Inclusion Criteria:

A. Adults with hormone receptor-positive (HR+) early breast cancer were confirmed.

B. At least 18 years old at the time of signing the informed consent. C. Candidates for adjuvant endocrine therapy in combination with Abemaciclib (either NHI-reimbursed or self-funded), as determined by the primary treating physician. Prior chemotherapy requirement: Must have completed prior adjuvant chemotherapy at least 21 days before enrollment. All chemotherapy-related toxicities (except alopecia or Grade 2 peripheral neuropathy) must have resolved to ≤ Grade 1.

D. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.

E. Adequate bone marrow function as defined by:

1. Absolute Neutrophil Count (ANC) ≥ 1,500/μL without G-CSF support within 14 days of enrollment.
2. Platelet count ≥ 100,000/μL.
3. Hemoglobin ≥ 8.0 g/dL (red blood cell transfusion is permitted prior to enrollment).

F. Adequate hepatic function, as defined by:

1. Serum alkaline phosphatase (ALP), aspartate aminotransferase (AST), and alanine aminotransferase (ALT) ≤ 3× the upper limit of normal.
2. Total bilirubin (T-bil) ≤ 1.5× the upper limit of normal. G. Willingness to refrain from using any other oral probiotics or prebiotics from the first dose of the study intervention ("Synbiotic Supplement" or placebo) through study completion.

H. Ability to communicate with study personnel and willingness to comply with study visits, treatment plans, and other trial regulations.

Exclusion Criteria:

A. Currently pregnant or breastfeeding. B. Received systemic antibiotic therapy for active viral, bacterial, or fungal infections within 2 weeks before the first dose of the study intervention.

C. Severe pre-existing medical conditions that, in the investigator's judgment, preclude participation, including but not limited to the following:

1. Severe renal impairment (e.g., estimated creatinine clearance < 30 mL/min).
2. Interstitial lung disease.
3. Severe dyspnea at rest or the requirement for oxygen therapy.
4. History of major gastrointestinal surgeries, such as gastrectomy or small bowel resection.
5. Pre-existing Crohn's disease, ulcerative colitis, or other chronic conditions that cause clinically significant diarrhea.

D. History of the following cardiovascular conditions: syncope of cardiovascular origin, ventricular arrhythmias of pathological origin (including ventricular tachycardia and ventricular fibrillation), or sudden cardiac arrest. Exception: Patients with atrial fibrillation that is controlled and stable for at least 30 days before randomization are eligible.

E. Unwillingness or inability to provide written informed consent.