This is a phase 2 randomized placebo-controlled crossover trial to determine the safety and efficacy of atomoxetine for treating obesity caused by loss-of-function variants in the melanocortin-4 receptor (MC4R), the most common cause of genetic obesity disorders. Atomoxetine was selected for this pilot trial because it has been shown to increase brain-derived neurotrophic factor (BDNF) within the central nervous system and in peripheral circulation. Targeting BDNF is a specific strategy for treating MC4R abnormalities because BDNF functions as a downstream mediator of MC4R signaling.