A Pilot Study of Metformin to Reduce Cerebrovascular Dysfunction in Participants With HIV and Metabolic Syndrome.

Inclusion Criteria: * Adult (\>18 years); * HIV+ on antiretroviral therapy for at least 12 months prior to study entry; * Viral load ≤50 copies/mL; * Able to provide informed consent; * Metabolic syndrome as per the harmonized criteria; * Women of child-bearing potential willing to use adequate contraception (defined as either an intra-uterine contraceptive device or hormonal contraceptive); Exclusion Criteria: * Treated with metformin as part of care; * History of drug or alcohol abuse within 3 months before screening; * Known neurosyphilis; * Known vitamin B12 deficiency; * Known neuropsychiatric disorders or serious psychiatric symptoms; * Significant head trauma with imaging structural abnormalities; * Renal impairment (estimated glomerular filtration \< 60 mL/min/1.73m2); * Type I or type II diabetes (fasting plasma glucose \>7mmol/L and/or HbA1c \>6.5%); * Hypersensitivity to metformin; * Any contraindication or special precaution in the metformin package insert which may put the participant at a safety risk; * Cationic drugs (as listed in the metformin package insert) that may increase metformin concentrations significantly; * Claustrophobia, metal implants or any other condition that prevents performing MR scan; * Pregnant / breastfeeding;