Efficacy and Safety of Adjuvant Metformin for Operable Breast Cancer Patients

Inclusion Criteria: * Operable Breast cancer patients with BMI ≥ 23 or 100 ≤ FBS \< 126 * From 6 month to 2 years since breast cancer operation and at least 4 weeks since chemotherapy or radiotherapy * No drug use except Tamoxifen * Normal OTPT \& Serum creatinine (\<=ULN) * ECOG performance status 0-2 or Karnofsky PS 60-100% * Life expectancy \> 12 * Absolute neutrophil count ≥ 1.5 x 10\^9/L * Platelets ≥ 100 x 10\^9/L * Pregnancy(-) \& without plan for pregnancy * Sign a written informed consent form Exclusion Criteria: * Type I or II DM or concurrent use of DM control agents * Prior use of Metformin * Hypoglycemia (FBS\< 70 with clinical symptom) * Concurrent investigational or commercial agents * Other diet or drug intervention for weight loss * Concurrent use of steroid * Abnormal liver and/or renal function * Symptomatic congestive heart failure / Cardiac arrhythmia / Angina pectoris * Ongoing or active infection * lactic acidosis * Pregnancy or ongoing breast feeding * Anorexia, bulimia, nausea due to other disease for longer than 1 month * Allergies or allergic reactions attributed to oral medications * Inability to swallow or digest oral medications * Physical or psychiatric illness that would limit compliance with study protocol * Participants in other clinical trial