• Participants with oral leukoplakia or erythroplakia with mild, moderate, or severe histologic dysplasia or hyperplasia at the high risk sites (e.g., floor of mouth, tongue). Lesions arising from the radiation field are excluded as study lesions.

• Measurable disease - minimum lesion size of 8x3 mm before initial biopsy

• Age \>= 21 years. Adults 18-20 are not included as Canadian law prohibits purchase of cigarettes under the age of 21; investigators wish to keep criteria consistent among all trial sites. Also, smokers aged \< 20 years would most likely not have oral leukoplakia

• Current and former smokers (\>= 5 packs in the lifetime)

• Karnofsky performance scale \>= 70%

• Leukocytes \>= 3,000/microliter

• Absolute neutrophil count \>= 1,000/microliter

• Platelets \>= 100,000/microliter

• Total bilirubin =\< 1.5 x institutional upper limit of normal (ULN)

• Aspartate aminotransferase (AST) (serum glutamic-oxaloacetic transaminase \[SGOT\]) and alanine aminotransferase (ALT) (serum glutamate pyruvate transaminase \[SGPT\]) =\< 1.5 x institutional ULN

• Estimation glomerular filtration rate (eGFR) \> 45 mL/min (eGFR calculated using the equation Chronic Kidney Disease Epidemiology Collaboration \[CKD-EPI\] creatinine)

• Willing to use adequate contraception (barrier method, abstinence, subject or partner has had a vasectomy or partner is using effective birth control or is postmenopausal) for the duration of study participation because the effects of metformin on the developing human fetus are unknown even though it is not teratogenic in rats and rabbits at 2-6 times the maximum recommended human daily dose. 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.

• Human immunodeficiency virus (HIV)-infected patients on effective anti-retroviral therapy with undetectable viral load within 6 months are eligible for this trial

• For patients with evidence of chronic hepatitis B virus (HBV) infection, the HBV viral load must be undetectable on suppressive therapy, if indicated

• Patients with a history of hepatitis C virus (HCV) infection must have been treated and cured. For patients with HCV infection who are currently on treatment, they are eligible if they have an undetectable HCV viral load

• Patients on chronic suppressive antiviral therapy for herpes simplex virus (HSV) are eligible

• Ability to take oral medication

• Ability to understand and the willingness to sign a written informed consent document in English or Spanish