Metformin Treatment in Infants After Perinatal Brain Injury

Inclusion Criteria: HIE Patients: 1. \> 35 weeks gestation at time of birth 2. ≤3 months at time of consent 3. Clinical diagnosis of HIE 4. Infant received therapeutic hypothermia for the treatment of HIE 5. Family lives within one hour distance of the Hospital for Sick Children (in order to facilitate home visits) Preterm Infants: 1. \<32 weeks gestation at time of birth 2. 36-44 weeks corrected (postmenstrual) gestational age at time of metformin administration 3. Clinical team anticipates hospitalization at SickKids for at least 1 week after study enrollment Exclusion Criteria: 1. Have a known genetic or chromosomal disorder. 2. Congenital or acquired liver or kidney disease that might, in the opinion of the Principal Investigator or delegate, affect drug metabolism. 3. History of hypoglycemia in the newborn period requiring glucose infusion rate \> 10 mg/kg/min or treatment with glucagon or diazoxide. 4. Any condition or diagnosis, that could in the opinion of the Principal Investigator or delegate, interfere with the participant's ability to comply with study instructions, might confound the interpretation of the study results, or put the participant at risk 5. \>3 months of age at the time of enrollment (term HIE patients only) 6. Weight \<10%ile based on WHO growth charts at time of initiation of study drug (term HIE patients only). 7. Maternal use of metformin while actively breastfeeding.