Purpose Two phase 2 studies evaluated the efficacy and tolerability of centanafadine sustained-release (SR) for adults with attention-deficit/hyperactivity disorder (ADHD). Patients and Methods In a phase 2a, flexible-dose, single-blind study, 41 male patients (aged 18‒55 years) with a diagnosis of ADHD (based on Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition) were titrated with centanafadine-SR 200‒300, 400, or 500 mg/d for 2 weeks, and then were treated with the titrated dose for 2 weeks. In a phase 2b, randomized, double-blind, placebo-controlled, crossover study, 85 male and female patients (aged 18‒60 years) with a diagnosis of ADHD (Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition) were titrated to target doses of centanafadine-SR 400, 500, 600, or 800 mg/d over the course of 1 week, and then received their titrated dose for 3 weeks. The primary outcome in both studies was mean total ADHD Rating Scale-IV (ADHD-RS-IV) score. Results In the phase 2a study, mean ADHD-RS-IV total score decreased by 21.41 (standard deviation 10.74) from the start of active centanafadine-SR treatment to the end of week 4 (P