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