Trial of Metformin for Colorectal Cancer Risk Reduction for History of Colorectal Adenomas and Elevated BMI

Inclusion Criteria:

* History of prior colorectal adenomas within the past 3 years; only patients who have had adenomas endoscopically removed are eligible; documentation of colorectal adenomas must be determined via review of pathology reports
* Body mass index (BMI) \>= 30; rounded to the nearest whole integer
* Eastern Cooperative Oncology Group (ECOG) performance status 0-1
* Leukocytes ≥ 3,000/μL (\>= 2,500/μL for African-American participants)
* Absolute neutrophil count \>= 1,500/μL (\>= 1,000/μL for African-American participants)
* Platelets \>= 100,000/μL
* Total bilirubin within normal institutional limits
* Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase \[SGOT\])/alanine aminotransferase (ALT) (serum glutamate pyruvate transaminase \[SGPT\]) =\< 1.5 X institutional upper limit of normal (ULN)
* Creatinine within normal institutional limits
* 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
* A serum pregnancy test must be performed and be negative in all women of childbearing potential within 2 weeks prior to starting treatment
* Ability to understand and willingness to sign a written informed consent document

Exclusion Criteria:

* History of colorectal cancer or other cancer(s) (except for non-melanoma skin cancers) within the last 3 years
* Family history of hereditary intestinal polyp disorder (e.g., familial adenomatous polyposis \[FAP\], hereditary non-polyposis colorectal cancer \[HNPCC\], Putz-Jegher's disease)
* Participants with diabetes
* History of vitamin B12 deficiency or megaloblastic anemia
* History of lactic acidosis
* Diet or other medications for weight loss
* Diseases associated with weight loss: anorexia, bulimia, or nausea
* Treatment with medications that may increase metformin levels: cationic drugs, e.g., digoxin, amiloride, procainamide, trimethoprim, vancomycin, triamterene, and morphine
* Treatment with other oral hypoglycemic agents
* Participants who have undergone full bowel resection, ablation or other local therapies
* Participants may not be receiving any other investigational agents
* History of allergic reactions attributed to compounds of similar chemical or biologic composition to metformin
* Participants with human immunodeficiency virus (HIV), cirrhosis of any cause, NASH (nonalcoholic steatohepatitis), or hepatitis (auto-immune or infectious)
* Kidney disease or renal insufficiency (defined as serum creatinine \> 1.4 mg/dL for females or \> 1.5 mg/dL for males)
* Metabolic acidosis, acute or chronic, including ketoacidosis
* Uncontrolled intercurrent illness including, but not limited to:

  * Ongoing or active infection
  * Symptomatic congestive heart failure
  * Unstable angina pectoris
  * Cardiac arrhythmia
  * Psychiatric illness/social situations that would limit compliance with study requirements
  * Renal failure
  * Hepatic failure
  * Sepsis
  * Hypoxia
* Pregnant or breastfeeding women are excluded
* Participants anticipating elective surgery during the study period
* Contraindication to colonoscopy/flexible sigmoidoscopy
* Participants may not be using metformin, cimetidine (Tagament) furosemide (Lasix), nifedipine (Cardizem), Ranitidine (Zinetac or Zantac), digoxin (Lanoxin), Quinidine or any other drug contraindicated for use with metformin
* Chronic alcohol use or a history of alcohol abuse
* Participants with any medical psychosocial condition that, in the opinion of the investigator, could jeopardize participation in and compliance with the study criteria
* Participants that regularly use aspirin (ASA), nonsteroidal anti-inflammatory drugs (NSAIDs), calcium, and cyclooxygenase (Cox)-2 inhibitors are not eligible for enrollment; however, patients that use aspirin 81 mg daily, or aspirin 325 mg, NSAIDs, calcium, or Cox-2 inhibitors at a frequency \< 10 times per month are eligible