Key PointsQuestionIs adolescents’ use of stimulant therapy for attention-deficit/hyperactivity disorder (ADHD) associated with cocaine and methamphetamine use as young adults?FindingsIn this cohort study of 5034 respondents, adolescents who used stimulant therapy for ADHD did not differ from their peers in cocaine and methamphetamine use at 19 to 24 years of age. In contrast, adolescents’ prescription stimulant misuse was associated with greater adjusted odds of later cocaine and methamphetamine use compared with population controls.MeaningThese findings suggest that adolescents’ use of stimulant therapy was not associated with increased risk for illicit stimulant use; however, prescription stimulant misuse was a signal for cocaine or methamphetamine use and warrants monitoring and screening.