The Safety and Efficacy of OPC-64005 in the Treatment of Adult Attention-deficit/Hyperactivity Disorder

Inclusion Criteria (Screening): * Male and female participants 18 to 55 years of age, inclusive, at the time of informed consent. * Participants with a primary Diagnostic and Statistical Manual of Mental Disorders, 5th edition (DSM-5) diagnosis of ADHD (including predominantly inattentive presentation, hyperactive presentation, and combined presentations) as confirmed by the Adult ADHD Clinical Diagnostic Scale (ACDS) v 1.2. * Participants willing to discontinue all prohibited psychotropic medication starting from the time of signing the informed consent and up to the 30 (+ 2)-day follow-up period. Exclusion Criteria: * Participants with a history of inadequate response or suboptimal tolerability to atomoxetine. * Participants who report allergies (lifetime treatment history) to stimulant or nonstimulant ADHD medications. * Participants with other DSM-5 disorders including psychosis (current or lifetime), bipolar disorder (current or lifetime), current major depressive disorder, or current panic disorder; or another psychiatric diagnosis that the investigator believes is primary or that will confound efficacy or safety assessments of the trail or interfere with participation in the trial otherwise. * Participants with a clinically significant current DSM-5 diagnosis of borderline, antisocial, paranoid, schizoid, schizotypal, histrionic, narcissistic, avoidant, obsessive compulsive, or dependent personality disorders. * Participants who currently have clinically significant dermatological, neurological, hepatic, renal, metabolic, hematological, immunological, cardiovascular, pulmonary, or gastrointestinal disorders such as any history of myocardial infarction, congestive heart failure, HIV (human immunodeficiency virus) seropositive status/acquired immunodeficiency syndrome, or active or chronic hepatitis B or C. * Participants with a history of obstructive sleep apnea.