Accumulating evidence suggests that metformin, used as an antidiabetic drug, possesses anticancer properties. Metformin reduced the incidence and growth of experimental tumors in vivo. In a randomized clinical trial among nondiabetic patients, metformin treatment significantly decreased the number of aberrant crypt foci compared to the untreated group with a follow-up of 1 month. In our study, HT29 cells were treated with graded concentrations of metformin, 10 mM/25 mM/50 mM, for 24/48 hours. We performed immunofluorescence experiments by means of confocal microscopy and Western blot analysis to evaluate a panel of factors involved in apoptotic/autophagic processes and oxidative stress response. Moreover, HT29 cells treated with metformin were analyzed by flow cytometry assay to detect the cell apoptosis rate. The results demonstrate that metformin exerts growth inhibitory effects on cultured HT29 cells by increasing both apoptosis and autophagy; moreover, it affects the survival of cultured cells inhibiting the transcriptional activation of nuclear factor E2–related factor 2 (NRF-2) and nuclear factor–kappa B (NF-κB). The effects of metformin on HT29 cells were dose- and time-dependent. These results are very intriguing, since metformin is emerging as a multifaceted drug: it has a good safety profile and is associated with low cost, and it might be a promising candidate for the prevention or treatment of colorectal cancer.