Patient description: A 42-year-old healthy woman came to the clinic to have her flu shot in early October. She works in a rehab center and has no underlying disease. It's her first time getting the vaccine this year. She is married for 5 years and uses barrier methods of contraception. Her menstrual cycle is irregular. She does not smoke. She is not on any medications. She exercises regularly for 30 minutes a day at least 5 days a week. She has no history of allergies to any food or drugs. Her past surgical history is significant for tonsillectomy and she is otherwise healthy.

Clinical trial eligibility criteria: eligible ages (years): 21.0-75.0,  Stage 0 Breast Carcinoma Breast Neoplasms Stage I Breast Carcinoma Stage II Breast Carcinoma Stage III Breast Carcinoma History of histologically-confirmed stage 0, I, II, or III breast carcinoma without evidence of disease at trial entry. Participants with a resected local recurrence are eligible Minimum of 6 months since last chemotherapy, biologic therapy (i.e., trastuzumab), radiation therapy, and/or breast surgery and no evidence of recurrent disease Minimum of 6 months since completion of adjuvant tamoxifen (tamoxifen is known to lower mammographic density119-121). Current use of a third generation aromatase inhibitor [AI] (i.e., anastrozole, letrozole, exemestane) is permitted provided that the participant has been on a stable dose for the past 6 months Age 21 to 75 years. Both pre and postmenopausal women will be included in this study. We will perimenopausal women, defined as menstrual cycle irregularity (variable cycle length that differs from normal by more than 7 days) and an Follicle-Stimulating Hormone (FSH) greater than 20 mili international units Negative serum pregnancy testing Normal mammogram of the contralateral breast within the past 12 months, defined as no new suspicious calcifications or other abnormal findings warranting a breast biopsy Eastern Cooperative Oncology Group (ECOG) performance status less than 2 Participants must have normal organ and marrow function within 28 days prior to randomization Body mass index (BMI) greater than or equal to 25 kilograms per meter squared or baseline fasting glucose of less than 126 milligrams per deciliter  History of histologically-confirmed bilateral breast cancer History of or plans for bilateral mastectomies Evidence of metastatic breast cancer Prior radiation therapy or implant in the contralateral breast Known diabetes (type 1 or 2) or baseline fasting glucose greater than 126 milligrams per deciliter Any 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, intake of 3 or more alcoholic beverages per day on average over the past 6 months) Currently taking metformin, sulfonylureas, thiazolidinediones, or insulin for any reason History of allergic reactions attributed to compounds of similar chemical or biologic composition to metformin or omega-3 fatty acids Receiving hormone replacement therapy, tamoxifen, or raloxifene within 6 months of trial entry Participants may not be receiving any other investigational agents for 30 days prior to baseline evaluation and during the study intervention