 Familial adenomatous polyposis FAP leads to adenomas and eventual adenocarcinomas in colon and less frequently, duodenum. Chemopreventive strategies have been studied in FAP patients to delay the development of adenomas and cancers. The non-steroidal anti-inflammatory drugs NSAIDs and selective cyclooxygenase-2 inhibitor have shown the regression of colorectal and duodenal adenomas in FAP patients. However, these drugs showed gastrointestinal damage and cardiovascular risks, and new preventive strategies are needed. Metformin, a biguanide, which is widely used for treating diabetes mellitus, has recently been suggested to have a suppressive effect on tumorigenesis via mTOR-inhibiting pathway, and have no significant safety issues in long term use. The investigators devised a double-blind randomized controlled trial to evaluate the effect of metformin on polyps of colorectum and duodenum in non-diabetic FAP patients. 