Effect of Vitamin D Supplementation on Chemotherapy Side Effects After Adjuvant Chemotherapy for Breast Cancer

This trial is testing whether vitamin D supplements can reduce the side effects of adjuvant chemotherapy (chemotherapy given after surgery) in breast cancer patients with low vitamin D levels. **You may be eligible if...** - You are between 18 and 80 years old - You have been diagnosed with breast cancer confirmed by biopsy - Your vitamin D blood level is below 20 ng/mL - You are scheduled to receive at least 4 cycles of adjuvant chemotherapy after surgery - Your general health is adequate (ECOG 0-2) **You may NOT be eligible if...** - You have had prior chemotherapy for breast cancer - You are allergic to vitamin D or calcium - You have conditions affecting vitamin D or bone health - You have taken vitamin D or calcium supplements in the past 3 months - You have other cancers - You are pregnant or breastfeeding Talk to your doctor to see if this trial is right for you.