The study is to evaluate the therapeutic effect of metformin as a maintenance therapy in high risk patients with complete remission of diffuse large B lymphoma / stage III follicular lymphoma after chemotherapy in the initial R-CHOP (cyclophosphamide, doxorubicin, vincristine, and prednisone, plus the monoclonal antibody rituximab) regimen

Inclusion Criteria:

1. Male or female patient age >14 years old;
2. Pathological types include: diffuse large B lymphoma, and stage III follicular lymphoma;
3. At the initial stage of therapy, received standard R-CHOP regimen and complete remission after the first course of treatment;
4. After complete remission, not consider to receive hematopoietic stem cell transplantation or chimeric antigen receptor T cell immunotherapy;
5. Serum glutamic-oxaloacetic transaminase (SGOT) and serum glutamic pyruvic transaminase (SGPT) not exceed 2 times of the normal upper limit;
6. Serum total bilirubin and creatinine not exceed 1.5 times of the normal upper limit;
7. Serum creatinine not exceed 1.5mg/dl;
8. Patients with high risk factors, including age > 60 year, or with diabetes/ impaired glucose tolerance, or with International Prognostic Index (IPI) score ≥2.
9. Sign informed consent file.

Exclusion Criteria:

1. Past medical history of high doses of cytarabine, methotrexate and rituximab maintenance therapy;
2. Past medical history of any type of hematopoietic stem cell transplantation;
3. Past medical history of lactic acidosis;
4. Extreme weight loss failure, malnutrition or dehydration patients;
5. Pregnant women or lactating women, or women who do not take contraceptive measures for childbearing age;
6. Alopecia, mental retardation or psychiatric disorders that affect the patient's normal informed consent;
7. Type 1.2 diabetes with ketoacidosis, liver function or renal insufficiency, pulmonary insufficiency, heart failure, acute myocardial infarction, severe infection and trauma, major surgery and clinical hypotension or hypoxia;
8. Diabetes complicated with severe chronic complications (such as diabetic nephropathy, diabetic retinopathy);
9. Any other serious complications occurred, depending on the outcome of the study;
10. Before the intravenous pyelography or anterior angiography;
11. Alcoholics;
12. Deficiency of Vitamin B12, folic acid or iron.