The treatment period will begin following randomization. On day M0 baseline measurements will be taken and recorded, and metformin administration will be begun. In order to optimize the treatment tolerance, it will be suggested to the patients to take the pill during or at the end of the lunch. During the first week, the posology of the placebo and metformin will be 500 mg at the breakfast. After, the posology will be increased every week as follow: 500 mg morning and afternoon, then 1000 mg morning and afternoon (2000 mg per day). In case of intolerance, the maximum posology tolerated will be maintained. In fact regarding the primary data of the trial regarding the effect of metformin on colonic polyp, it seems possible that low dose of metformin are potentially active This treatment will continue until the end of the study.