The investigators hypothesize that adjuvant metformin use in breast cancer patients with overweight or pre-diabetes mellitus (DM) may improve their body condition including weight loss.

In this study, the investigators aim to test the efficacy and safety of adjuvant metformin for operable breast cancer patients with overweight or pre-DM.

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