Cytochrome P450 2D6 (CYP2D6) is an important enzyme in the body for breaking down many medications that are commonly used in children of various ages. The purpose of this proposal is to investigate the relative roles of development and genetic variation in CYP2D6 activity in school-aged children and adolescents with attention deficit and hyperactivity disorder and health controls using the over-the-counter cough suppressant, dextromethorphan or "DM", a standard probe for determining CYP2D6 phenotype. Embedded in the study design are sub-studies to search for by-products of normal body metabolism that reflect differences in enzyme activity, and a pharmacokinetic study to assess the consequences of CYP2D6 genetic variation on the systemic exposure to medications used by this patient population. Ultimately, the goal of the research is to personalize the use of medications in children by selecting the appropriate dose of the correct medication for individual patients.