Objective: To assess the efficacy and safety of AR19 in the treatment of attention-deficit/hyperactivity disorder (ADHD) diagnosed by DSM-5 criteria in adults from 18 through 55 years of age. AR19 is a pellets-in-capsule, immediate-release amphetamine sulfate investigational formulation with physical and chemical barriers designed to resist manipulation to deter snorting, smoking, and intravenous injection.Methods: This randomized, double-blind, placebo-controlled, fixed-dose, forced titration, multicenter trial investigated the safety and efficacy of AR19 from September 2018 to April 2019. Study participants were randomized and titrated to 20 mg or 40 mg AR19 daily or placebo. Study medication was dosed once in the morning and again 4 to 6 hours later for a period of 5 weeks. The primary efficacy measure was the total score on the Adult ADHD Investigator Symptom Rating Scale (AISRS).Results: Participants (N = 320) were randomized and received at least 1 dose of study medication. Demographics and baseline characteristics were similar across treatment groups. The least squares mean treatment differences versus placebo (97.5% CI) were -7.2 (-11.3 to -3.1) for the AR19 20-mg group and -7.3 (-11.4 to -3.2) for the AR19 40-mg group (each P < .001). The most common treatment-emergent adverse events occurring in participants in the AR19 treatment groups were insomnia, dry mouth, decreased appetite, palpitations, headache, and tachycardia and are consistent with the known safety profile of amphetamine sulfate.Conclusions: AR19 demonstrated efficacy on all endpoints and was generally well tolerated, supporting the efficacy and safety of AR19 20 mg and 40 mg in adults with ADHD.Trial Registration: ClinicalTrials.gov Identifier: NCT03659929.