Bone mass achieved by early adulthood is a major determinant of lifetime risk for osteoporosis. Therefore, optimizing peak bone mass is crucial to avoiding bone fracture with its associated morbidity and mortality. Emerging evidence suggests that serotonin plays a central role in bone metabolism. For example, preclinical experiments have shown that bone cells express the serotonin transporter and a variety of functional serotonin receptors whose activity modulates bone turnover. Epidemiologic studies have linked selective serotonin reuptake inhibitors (SSRIs) to reduced bone mineral density and increased fracture risk in the elderly. SSRIs are widely used in youths to treat a number of psychiatric disorders. However, while their short-term efficacy and safety have been established, their long-term safety remains little investigated. We, here, propose to recruit, in a 2-year prospective observational study, 15 to 20 year-old participants upon the initiation of SSRI treatment. During the period of the Award, bone mineral density of the lumbar spine and whole body will be measured using dual x-ray absorptiometry and of the radius using peripheral quantitative computerized tomography. A detailed psychiatric assessment will be conducted to control for psychopathology, as a potential confounding factor affecting bone mineralization. Changes in psychiatric treatment during the follow up period will also be documented and accounted for. By using a group of healthy controls, of comparable age and sex distribution, we aim to evaluate 1) whether psychopathology, at baseline, is associated with low bone mass, 2) if treatment with SSRIs suppresses bone mineralization, and 3) if the discontinuation of the SSRI is followed by a restoration of bone mineral accrual. 4) Furthermore, genetic testing will investigate whether variants of the serotonin system genes moderate the effect of SSRI treatment on bone mineral density. In sum, this work aims to improve the long-term safety of psychiatric treatments in order to optimize functioning and the quality of life of those who suffer from psychiatric disorders. This is consistent with the mission of the National Institute of Mental Health.