Gastric intestinal metaplasia significantly increases the risk of gastric cancer. Metformin, a biguanide, which is widely used for treating diabetes mellitus, has recently been suggested to have a suppressive effect on tumorigenesis and cancer cell growth. The investigators devised a prospective randomized controlled trial to evaluate the chemopreventive effect of metformin against gastric intestinal metaplasia and the safety of this drug in non-diabetic gastric intestinal metaplasia patients.

Inclusion Criteria:

1. Patients aged from 18 to 75 years old;
2. Body mass index (BMI) ranged from 18.5 to 23.9 at enrollment;
3. IM patients with OLGIM stage Ⅱ-Ⅲ diagnosed by upper gastrointestinal endoscopy and histopathological biopsy within the last 3 months;
4. Patients without H. pylori infection confirmed by 13C-urea breath test (UBT) or patients with H. pylori infection who completed the bismuth-containing quadruple program and had confirmed successful eradication by 13C-UBT.

Exclusion Criteria:

1. History of diabetes mellitus (use of medication and/or HbA1c over 6.5%);
2. History of regular use (defined as at least once per week) of NSAIDs and/or aspirin;
3. History of stomach surgery (including endoscopic submucosal dissection and endoscopic mucosal resection) or previously diagnosed malignant tumor;
4. History of heart failure, renal failure, liver cirrhosis or chronic hepatic failure;
5. Patients with contraindications or allergies to the drugs in this study;
6. Breastfeeding or pregnancy;
7. History of substance abuse or alcohol abuse in the past 1 year;
8. Severe mental illness;
9. Refusal of drug treatment;
10. Refusal of signing informed consent.