Epidemiologic studies have shown that diabetes mellitus is associated positively with increased risk of pancreatic ductal adenocarcinoma (PDAC), and recent meta-analysis studies showed that metformin, reduces the risk of pancreatic cancer (PC). We tested the effects of metformin on pancreatic intraepithelial neoplasia (PanIN) and their progression to PDAC in p48Cre/+.LSL-KrasG12D/+ transgenic mice. Mice fed control diet showed 80% and 62% incidence of PDAC in males and females, respectively. Male mice showed 20% and 26%, and female mice showed 7% and 0% PDAC incidence with 1000- and 2000-ppm metformin treatments, respectively. Both doses of metformin decreased pancreatic tumor weights by 34% to 49% (P < 0.03-0.001). The drug treatment caused suppression of PanIN 3 (carcinoma in situ) lesions by 28% to 39% (P < .002) and significant inhibition of carcinoma spread in the pancreas. The pancreatic tissue and/or serum of mice fed metformin showed a significant inhibition of mammalian target of rapamycin (mTOR), extracellular signal-regulated kinases (ERK), phosphorylated extracellular signal-regulated kinases (pErk), and insulin-like growth factor 1 (IGF-1) with an increase in phosphorylated 5' adenosine monophosphate kinase (pAMPK), tuberous sclerosis complex 1 (TSC1, TSC2), C-protein and an autophagy related protein 2 (ATG2). The cancer stem cell (CSC) markers were significantly decreased (P < 0.04-0.0002) in the pancreatic tissue. These results suggest that biologic effects of metformin are mediated through decreased CSC markers cluster of differentiation 44 (CD44 and CD133), aldehyde dehydrogenase isoform 1 (ALDH1), and epithelial cell adhesion molecule (EPCAM) and modulation of the mTOR signaling pathway. Our preclinical data indicate that metformin has significant potential for use in clinical trials for PC chemoprevention.