Before beginning the study, a meeting between the study coordinator and the participant will occur in order to get consent and in order for the participant to ingest a test dose of MPH under the supervision of a nurse. The study coordinator will contact the participant in order to arrange a time for the approximately 4 hour meeting. The weight and height (thus BMI) will be measured upon arrival. The participant will then have to respond to questionnaires, e.g. the Wender-Utah Rating Scale (to assess traits compatible with ADHD), the Beck Depression Inventory (to assess depressive symptoms), the "Three-Factor Eating Questionnaire" (to assess eating style) and the Drug Abuse Screening Test (to ensure no underlying drug problem). If the participant is a female they cannot be pregnant to be included in the study; thus a pregnancy test will be administered and if it is determined that they are pregnant they will be referred to visit their family physician and excluded from the study. If eligible, the participant will be asked to read and sign the consent form. Eligible participants will then see a nurse who will administer the all of the measures outlined in the Case Report Form, e.g., a physical exam, medical history assessment, baseline vital signs, electrocardiogram (ECG), etc. The nurse will then administer a test dose of 0.5mg/kg of MPH and for a period of three hours after the ingestion of MPH the nurse will closely monitor any potential side-effects of the drug by measuring heart rate, blood pressure, and by following a detailed side-effects check-list. If at any point the participant rates and of the side effects as "Prohibitive", then he/she will be excluded from the study. The participant will also be excluded from entering the study if they demonstrate an abnormal ECG systolic or if blood pressure exceeds the baseline reading by 20mmHg or if diastolic blood pressure exceeds the baseline reading by 10mmHg, BP > 160/100, or resting pulse increased by 20 beats/minute from the baseline. As part of the comprehensive safety assessment to determine eligibility, the ECG data file that is collected will be stored via a USB external hard drive and then sent to CHEO cardiology to be read by a cardiologist. Once it is determined by the cardiologist that the ECG rhythm is normal, both the cardiologist and the QI will sign off on the Case Report form, indicating that the participant is eligible for enrollment. If it is determined by the ECG reading that the participant is not eligible, they will be called up by the study coordinator and told of the incidental findings and subsequently informed to visit their family physician to follow-up. The participant may bring the consent form home to further study its content before signing it. Prior to leaving the initial screening visit participants will be instructed on how to strap on an omni-directional accelerometer and instructed on how to fill out the log sheet which gathers information on when the monitors are worn and when they are taken off, which is needed to evaluate wear time for a seven day period. Participants will be instructed to return the accelerometers at their next visit, Baseline 1.