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 biop…