Atomoxetine in Melanocortin Obesity Syndrome

Inclusion criteria: * Age 6 years and above * Documented MC4R variant classified as pathogenic, likely pathogenic, or variant of uncertain significance per ACMG criteria. If testing was done in a research lab, it will be confirmed by a CLIA-approved lab prior to randomization. * Obesity defined as BMI ≥30 kg/m2 in adults or ≥95th percentile for age and sex in children Exclusion criteria: * Use of atomoxetine, viloxazine (another selective norepinephrine-reuptake inhibitor), methylphenidate, amphetamine, dextroamphetamine, lisdexamfetamine, phentermine, or any other stimulant medication in the past 30 days. If on other ADHD medications, such as guanfacine and clonidine, must be on a stable dose for \>3 months. * Weight loss \>5% in the past 3 months. * Initiation of new weight loss program, including diet or medications. If on weight loss medications, must be on a stable dose for \>3 months. * Inability to swallow capsules. * History of hypersensitivity to atomoxetine. * Narrow angle glaucoma. * History of pheochromocytoma. * Uncontrolled Stage 2 hypertension (≥95th percentile + 12 mmHg or \>140/90, whichever is lower) at screening. If on antihypertensive medication, must be on stable dose for \>3 months. * Hepatic insufficiency including cirrhosis and acute hepatitis (AST or ALT \>3x upper limit of normal) * Uncontrolled asthma requiring albuterol more than once weekly over the past 3 months * History of a cardiac arrhythmia (not including bradycardia) * Current use of monoamine oxidase inhibitors * Pregnancy or intention to become pregnant during the next year * History of Major Depressive Disorder in the past 2 years, lifetime history of suicide attempt, history of any suicidal behavior in the past month, history of other severe psychiatric disorders (e.g. schizophrenia, bipolar disorder) * PHQ-9 score is ≥15 or suicidal ideation of type 4 or 5 (C-SSR) in the past month * Unable to comply with study procedures in the opinion of the investigator