A Phase II Trial Assessing the Tolerability of Palbociclib in Combination With Letrozole or Fulvestrant in Patients Aged 70 and Older With Estrogen Receptor-Positive, HER2-Negative Metastatic Breast Cancer

• Documentation of disease: estrogen receptor positive and/or progesterone receptor (PR) positive, HER2 negative metastatic breast cancer; histologic confirmation is required

• Measurable disease or non-measurable disease

• Planning to begin palbociclib for metastatic disease; one prior line of endocrine therapy and/or chemotherapy for metastatic disease is allowed; patients may begin or have already begun endocrine therapy before they start palbociclib treatment, but no more than two weeks prior to registration

• No prior therapy with a CDK inhibitor

• Resolution of all acute toxic effects of prior therapy or surgical procedures to CTCAE grade =\< 1 (except alopecia) or to baseline toxicities prior to previous therapy or surgical procedures, prior to registration

• No untreated brain metastases; patients with treated brain metastases must have completed treatment with steroids to be eligible

• No known interstitial lung disease

• No second malignancies other than non-melanoma skin cancers or cervical carcinoma in situ; however, patients are not considered to have a currently active malignancy if they have completed therapy and are free of disease for \>= 3 years

• No active infection requiring treatment with antibiotics

• Patients must be able to swallow and retain oral medication

• Eastern Cooperative Oncology Group (ECOG) performance status 0, 1 or 2

• Patients must be able to read and comprehend English or Spanish

• Absolute neutrophil count (ANC) \>= 1500/mm\^3 (1.5 x 10\^9/L)

• Platelet count \>= 100,000/mm\^3 (100 x 10\^9/L)

• Creatinine clearance \>= 30 ml/min calculated using the Cockcroft-Gault formula

• Total serum bilirubin =\< 1.5 upper limit of normal (ULN) (\< 3 ULN if Gilbert's disease)

• Aspartate aminotransferase (AST) and/or alanine aminotransferase (ALT) =\< 3 x ULN (=\< 5.0 x ULN if liver metastases present)

• Alkaline phosphatase =\< 2.5 x ULN (=\< 5 x ULN if bone or liver metastases present)