Study Day 1 (about 5 hours long) Within approximately one week of the pre-study visit, subjects will be admitted to the study unit. The child must refrain from eating 2 hours prior to the scheduled visit. Prior to dosing, the subjects will completely empty their bladders and provide a blank urine sample. Each subject will then be given a single 0.5 mg/kg dose of DM as Robitussin Cough (7.5 mg/5 ml; alcohol-free, fruit punch flavor). All urine produced over the 4-hour study period will be collected. Urine will be retained for analysis of DM and its three metabolites, and for metabolomic analyses. One hour after dosing, the subjects will be provided with a small meal that they will select from the standard hospital menu. A pregnancy test will be performed for females of child bearing age. Subjects will be asked to provide breaths (inspiratory and expiratory) for ten minutes with a handheld indirect calorimeter (ie. MedGen/Body Gem: also known as "GEM") with a clip on their nose. The measurements obtained for resting metabolic rates will occur prior to dosing and 4 hours post dose.