This randomized controlled Phase III trial was designed to evaluate the effect of vitamin D supplementation (VD) on chemotherapy side effects following adjuvant chemotherapy in breast cancer

This is a parallel group, open-label randomized controlled trial to investigate the occurrence of side effects of vitamin D supplementation (VD) in adjuvant chemotherapy in patients with breast cancer. Both groups will receive standard adjuvant chemotherapy on day 1 and each subsequent cycle. In addition, vitamin D2 will be given randomly to both groups. Blood samples and imaging results were collected and analyzed before initiation of adjuvant chemotherapy and after every two cycles. The primary outcome to be documented was associated grade III or higher adverse events during adjuvant chemotherapy. Primary and secondary study findings and adverse events will be thoroughly evaluated.

Inclusion Criteria:

1. Age between 18 and 80 years, with an Eastern Cooperative Oncology Group (ECOG) performance status of 0-2.
2. Primary diagnosis of breast cancer confirmed by preoperative pathological examination.
3. Serum 25(OH)D levels less than 20 ng/ml (50 nmol/L).
4. Patients who have not previously received chemotherapy and who plan to undergo at least 4 cycles of
5. Patients with adjuvant chemotherapy or combined targeted therapy.
6. Life expectancy of at least 6 months.
7. No other uncontrolled benign diseases at the time of recruitment.
8. All patients must have complete clinical medical records.
9. Willingness to voluntarily sign an informed consent form.

Exclusion Criteria:

1. History of invasive breast cancer.
2. Prior systemic treatment for the treatment or prevention of breast cancer.
3. Known allergic reactions to vitamin D or calcium compounds.
4. Comorbidities that may affect vitamin D or calcium balance or bone health.
5. Vitamin D or calcium supplementation in the past 3 months.
6. Presence of other tumors.
7. Pregnant or lactating women.
8. Individuals who do not wish to participate in the study.