Metformin Prostate Cancer Adjuvant Trial

Inclusion Criteria: * Male \> 18 years of age * Biopsy confirmed adenocarcinoma of the prostate * Able to swallow and retain oral medication * Hemoglobin A1C \<7.0% * Able and willing to participate in the full 12 months of the study * Able to understand instructions related to study procedures * Able to read and write English Exclusion Criteria: * Metastatic Prostate Cancer * Prostate Specific Antigen Doubling Time \< 6 months * Prior Chemotherapy, hormonal therapy, oral glucocorticoid therapy, Gonadotropin-releasing hormone analogue therapy * Current or previous use of 5α-reductase inhibitors, antiandrogen drugs, metformin, oral or injectable diabetes drug * Diagnosis of Type 1 Diabetes Mellitus * Known hypersensitivity to metformin * any condition associated with increased risk of metformin associated lactic acidosis * participation in any investigational or marketed drug trial within 30 days prior to screening or during study period * any unstable, serious co-existing medical conditions including but not limited to myocardial infarction, coronary bypass surgery, unstable angina, cardiac arrhythmias, clinically evident congestive heart failure, or cerebrovascular accident within 6 months prior to screening * history of megaloblastic anemia * abnormal liver function test (total bilirubin, aspartate aminotransferase, alanine aminotransferase, alkaline phosphatase or serum creatinine above upper limit of normal) * history of other malignancies, with the exception of adequately treated non-melanoma skin cancer, stage 1 melanoma, non-muscle invasive bladder cancer, or other solid tumors curatively treated with no evidence of disease for at least 5 years * history or current evidence of substance abuse within 12 months of screening * history of any illness (including psychiatric) that, in the opinion of the investigator, might confound the results of the study or post an additional risk to the subject