Metformin Hydrochloride vs. Placebo in Overweight or Obese Patients at Elevated Risk for Breast Cancer

Inclusion Criteria:

* Patients must be post-menopausal women; post-menopausal women are defined as: (1) those \>= 50 years of age who had not menstruated during the preceding 12 months or who had castrate follicle-stimulating hormone levels (\> 40 IU/L), (2) those who had undergone a bilateral oophorectomy
* Patients must be at elevated risk for breast cancer based on strong family history or a history of breast biopsy documenting atypical hyperplasia anytime in the past; for this study strong family history is defined as having:

  * 1 first-degree (parent, offspring, sibling) relative =\< 50 years old when diagnosed with breast cancer, or
  * \>= 2 first-degree relatives of any age when diagnosed with breast cancer, or
  * \>= 2 second-degree (aunts, uncles, grandparents, grandchildren, nieces, nephews, or half-siblings) maternal or paternal relatives diagnosed with breast cancer and at least 1 diagnosed at =\< 50 years of age
* Patients must have a body mass index (BMI) \>= 25.0 as calculated by the formula: weight in pounds / height squared x 703 = BMI; a BMI of:

  * 18.5-24.9 is considered normal;
  * 25.0-29.9 is considered overweight;
  * 30.0+ is regarded as obese
* Patients must be willing to complete a bilateral mammogram at baseline with repeat exam after 12 cycles of protocol therapy; patients who have had a mammogram within 1 month prior to registration to protocol therapy will not need to repeat the exam
* Patients must be willing to provide a core tissue biopsy at baseline and with repeat tissue collection after 12 cycles of protocol therapy
* White blood cell (WBC) \>= 3.0 x 109/L
* Granulocytes (polymorphs + bands) \>= 1.5 x 109/L
* Platelets \>= 100 x 109/L
* Hemoglobin \>= 110 g/L
* Aspartate aminotransferase (AST) =\< 1.8 X upper limit of normal (ULN)
* Alanine aminotransferase (ALT) =\< 1.8 X ULN
* Alkaline phosphatase =\< 2 X ULN
* Serum creatinine =\< 115 umol/L (1.3mg/dL)
* Serum bilirubin =\< institution ULN (except for subjects with Gilbert's Disease who are eligible despite elevated serum bilirubin level)
* 12 hour fasting glucose level \< 7.0 mmol/L
* Eastern Cooperative Oncology Group (ECOG) performance status of 0 - 1 within 28 days of registration
* Life expectancy of \>= 5 years
* Subjects must be accessible for treatment, adverse event tracking and follow-up as determined by the treating physician
* Subject consent and authorization for the release of health information must be obtained according to local institutional guidelines

Exclusion Criteria:

* No history of any malignancy except: adequately treated non-melanoma skin cancer, curatively treated in-situ cancer of the cervix, or other solid tumors curatively treated with no evidence of disease for \>= 5 years
* No known diabetes (type 1 or 2) or baseline fasting glucose \>= 7.0 mmol/L
* No known hypersensitivity or intolerance to metformin
* No condition associated with increased risk of metformin-associated lactic acidosis (e.g. congestive heart failure defined as New York Heart Association \[NYHA\] class III or IV functional status, history of acidosis of any type; habitual intake of 3 or more alcoholic beverages per day)
* No current treatment with metformin, sulfonylureas, thiazolidinediones or insulin for any reason
* No breastfeeding
* No concurrent or planned participation in randomized trials of weight loss or exercise interventions or trials targeting insulin, insulin-like growth factor 1 (IGF-1) or their receptors