Switching Medications for Side Effects in Breast Cancer

Inclusion Criteria: * Histologic documentation of DCIS or invasive breast cancer by core needle or incisional biopsy. * The DCIS or invasive cancer must be estrogen receptor alpha (ER)-positive * The invasive cancer must be HER2-negative (IHC 0-1+, or with a FISH ratio of \<1.8 if IHC is 2+ or if IHC has not been done) * Clinical Stage I-III invasive breast cancer or DCIS * Patients for whom adjuvant treatment with one of the following would be clinically indicated: letrozole, anastrozole, exemestane * Patients who anticipate continuing with adjuvant endocrine therapy for a minimum of 2 years at the time of registration * Women over 18 years of age who are post-menopausal, defined as last menstrual period \>2 years prior to registration, or \>1 year prior to registration with FSH and estradiol in post-menopausal range. * Patients must meet the following clinical laboratory criteria: * Absolute neutrophil count (ANC) \>1,000/mm3 and platelet count \> 75,000/mm3 * Total bilirubin \<1.5 x the upper limit of the normal range (ULN). * Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) \< 3 x ULN. * Ability to give informed consent. Exclusion Criteria: * Prior endocrine therapy for any histologically-confirmed cancer or prevention of breast cancer in the last 10 years is not allowed. * Any other adjuvant therapy for breast cancer. Exception: Bisphosphonate or denosumab treatment for metabolic bone issues are allowed. * Patients who are prescribed tamoxifen as the first endocrine therapy rather than an aromatase inhibitor. * Women who are pregnant or lactating. * Prisoners.