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Metformin (MF), a diabetic drug, has antineoplastic activity as adjuvant therapy for breast cancer and prostate cancer. MF is thought to work via inhibition of mammalian target of rapamycin and activation of p53 and liver kinase B1 via adenosine 5′-monophosphate–activated protein kinase. We investigated survival, recurrences and metastasis in patients with type 2 diabetes mellitus (DM2) along with colorectal cancer (CC) or lung cancer (LC) taking MF using the electronic medical record in Memphis Veterans Affairs Medical Center (colon, n = 202; lung, n = 180).Patients with CC or LC and DM2 on MF were compared to controls taking any medication except MF. Recurrences, metastases, secondary cancers, survival and carcinoembryonic antigen levels were compared using t test and chi-squared test. Inclusion criteria were based on MF use, CC or LC diagnosis and DM2.For CC, the MF group noted fewer deaths (48% versus 76%, P < 0.001), recurrences (4% versus 19%, P = 0.002), metastases (23% versus 46%, P = 0.001), better 5-year survival rates (57% versus 37%, P = 0.004), overall survival years (5.7 versus 4.1, P = 0.007) and greater carcinoembryonic antigen decrease (72% versus 47%, P = 0.015). MF was associated with improved 5-year survival rates (29% versus 15%, P = 0.023) and overall survival years (3.4 versus 1.8, P < 0.001) in LC.Our study shows that MF therapy is associated with significantly better prognosis in patients with CC and improved survival in LC. Patients with CC on MF had fewer recurrences and metastases. Differences in metabolic pathways between CC and LC likely account for the differences in the effect of MF.