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