Metformin Therapy for Overweight Adolescents With Type 1 Diabetes

Inclusion Criteria: 1. Clinical diagnosis of presumed autoimmune type 1 diabetes (T1D) as indicated by age of diagnosis \<10 years or documented positive diabetes-related autoantibodies. a. Note: For randomization, presence of at least one of the diabetes-related autoantibodies \[Insulin autoantibodies (IAA) at diagnosis prior to initiation of insulin, Islet cell antibodies (ICA), Anti-GAD (GAD65), Anti-IA2 (IA2), Zinc Transporter 8 (ZnT8)\] must be documented either from medical records or new laboratory measurement (IAA and ICA not measured by central lab) sent to central lab for participants who were ≥10 years old at diagnosis. 2. Age: 12 to \<20 years. 3. Duration of type 1 diabetes: ≥1 years. 4. Current insulin regimen involves either use of an insulin pump or multiple daily injections of insulin (at least 3 shots per day) for the last three months, with no plans to switch the modality of insulin administration during the next 6 months (e.g., injection user switching to a pump, pump user switching to injections). 5. Hemoglobin A1c: 7.5% - \<10.0% from point of care measurement or local lab on day of screening visit or within 1 month prior. 6. BMI: ≥85th percentile adjusted for age and sex . 7. Total daily dose of insulin: ≥0.8 units per kg per day. 8. Average of ≥3 Self-Monitoring Blood Glucose (SMBG) tests per day prior to initiating study and from download of study-provided blood glucose meter following screening visit. 9. Available for at least 6 months of follow-up, has home phone (or access to phone), and willing to be contacted by clinical site staff. 10. Expected to comply with protocol in investigator's judgment. Exclusion Criteria: 1. Use of non-insulin medications for blood glucose control within prior 6 months or planning to use within next 6 months (other than study drug). 2. Use of medications for weight reduction (such as: Belviq (lorcaserin), Qsymia (Phentermine + topiramate), Orlistat (xenical)) within the prior 6 months or planning to use within next 6 months. 3. Use of a medication such as stimulants, psychotropic agents and oral/inhaled glucocorticoids that could affect weight gain or glycemic control of T1D or planning to use within the next 6 months. 4. Any condition that in the judgment of the investigator will adversely affect the completion of the protocol. 5. Females: pregnant, lactating, or intending to become pregnant within the next 34 weeks * A negative urine pregnancy test will be required for all females An effective contraceptive method or abstinence will be required for all females who have experienced menarche * Requirements regarding pregnancy testing prior to enrollment and monitoring for pregnancy over the course of the study may be further defined by each individual Institutional Review Board (IRB) 6. Clinical diagnosis of celiac disease that is in poor control as defined by most recent tissue transglutaminase (tTG) that is in the abnormal range. 7. History of ≥1 diabetic ketoacidosis events in the past 3 months. 8. History of ≥1 severe hypoglycemic events (cognitive impairment that required assistance to treat) in the past 3 months. 9. History of anemia or vitamin B12 deficiency in the past 2 years. 10. Participation in an intervention study in the past 3 months.