BACKGROUND AND OBJECTIVE: Viloxazine ER (extended-release capsules; Qelbree) is a nonstimulant medication that has been approved by the United States Food and Drug Administration (FDA) for treatment of pediatric and adult attention-deficit/hyperactivity disorder (ADHD). This phase 3, open-label extension (OLE) trial evaluated the long-term safety and efficacy of viloxazine ER in children and adolescents with ADHD. METHODS: Participants completing the phase 2 or one of the four phase 3 double-blind, placebo-controlled clinical trials were eligible for the OLE trial. Upon entering the OLE, double-blind treatment was discontinued and participants were administered viloxazine ER 100 mg/day (children, aged 6-11 years) or 200 mg/day, (adolescents, aged 12-18 years), with dosage titration as needed over a 12-week dose-optimization period (up to 400 mg/day [children] or 600 mg/day [adolescents]). Participants then entered a maintenance period that continued through US FDA-approval (up to 72 months). Safety (primary objective) was assessed relative to OLE baseline using adverse event (AE), clinical laboratory tests, vital sign, ECG, and Columbia Suicide Severity-Rating Scale (C-SSRS) monitoring. Efficacy was assessed relative to double-blind baseline using the ADHD Rating Scale (ADHD-RS-IV/5) and the Clinical Global Impression-Improvement (CGI-I) scale. Study visits for these assessments occurred every ~ 3 months throughout maintenance treatment. RESULTS: Participants (N = 1100) included 646 children and 454 adolescents (66.5% male/33.5% female). Median (range) exposure to viloxazine ER in the OLE was 260 (1-1896) days, and the median modal (most frequently used) viloxazine ER doses were 300 mg/day for children and 400 mg/day for adolescents. AEs included (≥ 5% incidence) nasopharyngitis (9.7%), somnolence (9.5%), headache (8.9%), decreased appetite (6.0%), and fatigue (5.7%). AEs were mostly mild or moderate in severity (3.9% reported any severe AE); AEs led to discontinuation in 8.2% of participants. The mean ± SD changes from double-blind baseline in ADHD-RS IV/5 total score were -24.3 ± 12.0 at Month 3, -26.1 ± 11.5 at Month 12, and -22.4 ± 13.6 at participants' last OLE study visit. CONCLUSIONS: The results of this large-scale safety trial support the long-term use of viloxazine ER as a generally well-tolerated and effective treatment option for pediatric ADHD. No new safety concerns emerged, and efficacy results suggest the potential for continued improvement over that seen during double-blind treatment. CLINICAL TRIAL REGISTRATION: Clinicaltrials.gov identifier: NCT02736656.