Metformin for Antipsychotic-induced Weight Gain in Adults With Intellectual Disability

Inclusion Criteria: * Stable outpatients * Age 16-65 years * Diagnosed with an IDD * On maintenance treatment with an antipsychotic (stable dose for ≥3 months). * BMI must be ≥30 kg/m2, OR ≥27 kg/m2 with at least one weight-related comorbidity (treated or untreated) such as: hypertension, dyslipidaemia, obstructive sleep apnea, or impaired fasting glucose, OR \>=25 for individuals who have gained \> 5% body weight in association with AP use. * Females of child-bearing age must be on one of the following regular contraceptives: 1. Agree to abstain from sex for the duration of the trial or 2. A barrier method of a diaphragm with spermicide and/or Latex condom or 3. An oral contraceptive agent, implantable contraceptive or an injectable contraceptive for at least six months prior to entering the study and will continue its use throughout the study, or 4. An intrauterine device, or 5. Partner has had a vasectomy at least 3 months prior to study start Exclusion Criteria: * Females who are nursing, currently pregnant, or have a positive pregnancy test * Clinical or laboratory evidence of uncompensated cardiovascular, endocrine, haematological, hepatic, renal, or pulmonary disease * Previous treatment and lack of efficacy or tolerability with metformin * History or diagnosis of Type 1 Diabetes (T1D) or Type 2 Diabetes (TD2) or fasting blood work, HbA1c \> 6.5% * History of metabolic acidosis or lactic acidosis * Treatment with weight-lowering agents * Medications with significant renal impact * Major medical or surgical event in the preceding 3 months * Acute suicidal risk. * Moderate to severe substance use disorder, other than caffein or nicotine use disorder