Background and Objectives: To evaluate the potential benefits of Bacteroides fragilis 839 (BF839), a next-generation probiotics, in reducing myelosuppression and gastrointestinal toxicity associated with chemotherapy in breast cancer patient. Methods and Study Design: 40 women with early breast cancer were randomly assigned to the BF839 (n=20) or placebo (n=20) during the administration of adjuvant chemotherapy (4 cycles of epirubicin 100mg/m^2 and cyclophosphamide 600mg/m^2 ). Myelosuppression and gastrointestinal adverse effects were monitored in both groups. Results: Throughout the four treatment cycles, the percentage of patients experiencing myelosuppression was 42.5% in the BF839 group, significantly lower than the 66.3% observed in the control group (p=0.003). Two patients in the BF839 group and three patients in the placebo group received recombinant human granulocyte colony-stimulating factor (rhG-CSF) due to leukopenia/neutropenia. When considering an ITT analysis, which included all patients regardless of rhG-CSF treatment, the BF839 group exhibited less reduction from baseline in white blood cells (-0.31±1.19 vs -1.15±0.77, p=0.012) and neutrophils (0.06±1.00 vs - 0.84±0.85, p=0.004) compared to the placebo group. The difference became even more significant when excluding the patients who received rhG-CSF injections. Throughout the four treatment cycles, compared to the placebo group, the BF839 group had significantly lower rates of 3-4 grade nausea (35.0% vs 71.3%, p=0.001), vomiting (20.0% vs 45.0%, p=0.001), and diarrhea (15.0% vs 30.0%, p=0.023). Conclusions: These findings suggest that BF839 has the potential to effectively mitigate myelosuppression and gastrointestinal toxicity associated with chemotherapy in breast cancer patients.