Therefore, the aim of this retrospective study was (1) to explore the association between the effectiveness and tolerability of atomoxetine in treating children with ADHD and plasma atomoxetine concentrations; (2) to provide potentially matched TDM reference range recommendations for atomoxetine in children receiving different dosing regimens based on their CYP2D6 genotypes. This evidence concerns the gene CYP2D6 and attention deficit-hyperactivity disorder.