The purpose of this study is to determine whether it is feasible to give a combination of Metformin and omega-3 fatty acids for one year to women with a history of early stage breast cancer. We will also evaluate whether the metformin and omega-3 fatty acids combination causes changes in breast tissue, blood, and mammograms.

Metformin is a medication used to treat and prevent diabetes and omega-3 fatty acids has been shown to lower cholesterol and improve cardiovascular health. Research has shown that Metformin and omega-3 fatty acids may also be effective in preventing cancer. In this study, we want to find out what effects, good and/or bad, the Metformin and omega-3 fatty acids combination has on you and your risk of developing a new breast cancer.

Inclusion Criteria:

• 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 exclude 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
• Participants must have a baseline mammographic density greater than or equal to 25 percent based upon the Breast Imaging Reporting and Data System (BIRADS) density score of 2, 3, or 4. Women with a baseline mammographic density of less than 25 percent (BIRADS Score= 1) will not be eligible
• Willingness to abstain from all omega-3 fish oil supplements for 30 days prior to baseline evaluation and during the study intervention
• Willingness to comply with all study interventions and follow-up procedures
• Ability to understand and willingness to sign a written informed consent document

Exclusion Criteria:

• 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
• Any omega-3 fatty acids should not be taken for 30 days prior to baseline evaluation and during the study intervention. If participants are consuming any of these items and would like to participate in this study, then a 30-day washout period will be required. - Uncontrolled or significant co-morbid illness patients receiving active chemotherapy or radiotherapy; or psychiatric illness/social situations that would limit compliance with study requirements