Primary: The primary objective of this study is to examine whether Methylphenidate (MPH) at dose of 0.50 mg/kg (not exceeding 100 mg/day) can lead to a sustained reduction in energy intake in obese adolescents and adults. Based on the investigators' pilot data in adults, it is predicted that daily administration of MPH for 60 consecutive days will produce reductions in food intake (both in lab and free-living) compared to placebo, extending our previous findings to a longer period of time and to obese adolescents. Prior to beginning the intervention, both MPH and placebo treatment groups will receive lifestyle advice (eating and exercise guidelines for healthy living). The primary outcome measures will rely on the validated measurements of energy intake (in lab lunch buffet and out of lab 3 day food menu).