Inclusion Criteria:  Former smokers (male and female) with a >= 20 pack year smoking history Quit smoking >= 12 months prior to enrollment Prostate, Lung, Colorectal and Ovarian Cancer Screening Trial (PLCOm2012) Lung Cancer Risk Prediction Score > 1.34  Overweight  Body mass index (BMI) > 25 and  Waist circumference  Female > 88 cm (35") Male > 102 cm (40") Age greater than 30 years. Participants younger than 30 years are unlikely to accrue enough smoking exposure followed by enough time after quitting (>12 months) to qualify. Eastern Cooperative Oncology Group (ECOG) performance status =< 1 Leukocytes >= 3,000/microliter Absolute neutrophil count (ANC) >= 1,500/microliter Platelets >= 100,000/microliter Total bilirubin < 1.5 x institutional upper limit of normal (IULN) Aspartate aminotransferase (AST) (serum glutamic-oxaloacetic transaminase [SGOT])/alanine aminotransferase (ALT) (serum glutamate pyruvate transaminase [SGPT]) =< 1.5 x IULN Estimated glomerular filtration rate (eGFR) > 45 ml/min (eGFR will be calculated using the equation Chronic Kidney Disease Epidemiology Collaboration [CKD-EPI] creatinine) Abnormal histology of at least one biopsy site on the baseline bronchoscopy defined as either metaplasia or dysplasia (mild, moderate, or severe). These correspond to a dysplasia score of 3-6 The effects of metformin ER on the developing human fetus at the recommended therapeutic dose are unknown. For this reason, women of child-bearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) prior to study entry and for the duration of study participation. Should a woman become pregnant or suspect she is pregnant while participating in this study, she should inform her study physician immediately. Ability to understand and the willingness to sign a written informed consent document.  Exclusion Criteria:  Current or previous diagnosis of diabetes mellitus (type I or type II diabetes) Use of metformin within the past 2 years Glycosylated hemoglobin A1C (HbA1c) > 8% History of allergic reactions attributed to compounds of similar chemical or biologic composition to metformin ER. Participants currently using immunosuppressive medication, including systemic steroids (not inhalational) and episodic use of inhaled steroids are excluded from this trial due to the potential impact of these treatments on the primary trial endpoint. Participants receiving any other investigational agents.  History of chronic alcohol use or abuse defined by any one of the following:  Average consumption of 3 or more alcohol containing beverages daily in the past 12 months Consumption of 7 or more alcoholic beverages within a 24 hour period in the past 12 months Acute or chronic liver disease, evidence of hepatitis (infectious or autoimmune), cirrhosis or portal hypertension. History of or current condition predisposing to increased risk for lactic acidosis such as: severe congestive heart failure (New York Heart Association [NYHA] class III or IV), metabolic acidosis, severe liver disease, or renal failure. Uncontrolled intercurrent illness that would raise concerns of safety or limit compliance with study requirements. Pregnant women are excluded from this study. Metformin ER is a class B agent that was not teratogenic in rats and rabbits at doses representing 3 and 6 times the maximum recommended human daily dose of 2000 mg; however, animal reproduction studies are not always predictive of human response. Because there is an unknown but potential risk for adverse events (AEs) in nursing infants secondary to treatment of the mother with metformin ER, breastfeeding should be discontinued if the mother is treated with metformin ER.