The aim of this study is to identify the effect of adjunctive metformin on recurrence of non-DM Stage II High-risk/ III colorectal cancer. This study is open-label randomized controlled study.

The primary endpoint is to compare the 3-year disease free survival between metformin group and non-metformin group. The secondary endpoint is to compare the 5-year overall survival and disease specific survival between two group, to identify the safety of metformin, and to compare the recurrence rate of polyps after polypectomy between two groups.

Inclusion Criteria:

1. patients with stage II high-risk/ III colorectal cancer who were taken an operation for cure, or patients with stage III rectal cancer who were taken an chemoradiation therapy and subsequently operation
2. age 20 - 80 years
3. patients without DM
4. patients who have well performance status for chemotherapy
5. patients who are agreed the study protocol

Exclusion Criteria:

1. patients with DM
2. patients who were diagnosed other cancers (≥ stage II) within 5 years
3. patients who have familial adenomatous polyposis, hereditary non-polyposis colorectal cancer, or inflammatory bowel disease
4. patients who are taken other chemoprevention drugs (ex. NSAIDs, Aspirin)
5. patients who have risk factors to increase lactic acidosis which are renal disease, lung disease, liver disease, or infectious disease
6. Creatinine ≥ 1.5mg/dL (for male), ≥ 1.4mg/dL (for female)
7. patients with congestive heart failure which is needed to treat
8. patients who have allergic history of metformin
9. pregnancy