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