To investigate if the widely publicized warnings in 2003 from the US Food and Drug Administration about a possible increased risk of suicidality with antidepressant use in young people were associated with changes in antidepressant use, suicide attempts, and completed suicides among young people. Quasi-experimental study assessing changes in outcomes after the warnings, controlling for pre-existing trends. Automated healthcare claims data (2000-10) derived from the virtual data warehouse of 11 health plans in the US Mental Health Research Network. Study cohorts included adolescents (around 1.1 million), young adults (around 1.4 million), and adults (around 5 million). Rates of antidepressant dispensings, psychotropic drug poisonings (a validated proxy for suicide attempts), and completed suicides. Trends in antidepressant use and poisonings changed abruptly after the warnings. In the second year after the warnings, relative changes in antidepressant use were -31.0% (95% confidence interval -33.0% to -29.0%) among adolescents, -24.3% (-25.4% to -23.2%) among young adults, and -14.5% (-16.0% to -12.9%) among adults. These reflected absolute reductions of 696, 1216, and 1621 dispensings per 100,000 people among adolescents, young adults, and adults, respectively. Simultaneously, there were significant, relative increases in psychotropic drug poisonings in adolescents (21.7%, 95% confidence interval 4.9% to 38.5%) and young adults (33.7%, 26.9% to 40.4%) but not among adults (5.2%, -6.5% to 16.9%). These reflected absolute increases of 2 and 4 poisonings per 100,000 people among adolescents and young adults, respectively (approximately 77 additional poisonings in our cohort of 2.5 million young people). Completed suicides did not change for any age group. Safety warnings about antidepressants and widespread media coverage decreased antidepressant use, and there were simultaneous increases in suicide attempts among young people. It is essential to monitor and reduce possible unintended consequences of FDA warnings and media reporting.