Adolescent patients transitioning from pediatric to adult medicine may experience patient safety risks. Using a large commercial insurance claims database, the authors compared opioid prescribing patterns and risk for opioid-related adverse events (overdose, opioid use disorder, or long-term use) among adolescents transitioning from “child” to “adult” at 18 years of age. The authors estimate a 14% increased risk for an opioid-related adverse outcome within one year when “adults” just over age 18 years were prescribed opioids that would not have been prescribed if they were under 18 years and considered “children.” The authors discuss how systematic differences in how pediatric and adult patients may be treated can lead to differences in opioid prescribing.