A Trial Evaluating the Efficacy, Safety, & Tolerability of Centanafadine Sustained-release Tablets in Adults With Attention-deficit/Hyperactivity Disorder

Inclusion Criteria: * Participants must meet the Diagnostic and Statistical Manual of Mental Disorders Fifth Edition (DSM-5) criteria for ADHD (including predominantly inattentive presentation, hyperactive presentation, or combined presentation) as confirmed by the Adult ADHD Clinical Diagnostic Scale (ACDS) Version 1.2. To confirm that ADHD is the primary diagnosis, the Mini International Neuropsychiatric Interview (MINI) will be used to identify and exclude other psychiatric conditions which would preclude enrollment. * Participants who were not receiving any pharmacological treatment for ADHD must have an Adult ADHD Investigator Symptom Rating Scale (AISRS) score of ≥ 28 at screening and baseline. Participants who were receiving pharmacological treatment for ADHD at screening must have a minimum AISRS score of ≥ 22 at screening, and a score of ≥ 28 at baseline. * All participants must be willing to discontinue all prohibited psychotropic medications starting from the time of signing the informed consent through the 7-day follow-up period. Participants that do not rollover into Trial 405-201-00015 (NCT03605849) must be willing to discontinue all prohibited psychotropic medications starting from the time of signing the informed consent until after the follow-up telephone call 10 days after the last dose of investigational medicinal product (IMP). * Participants must have a Clinical Global Impression-Severity of Illness Scale (CGI-S) score of ≥ 4 (≥ moderate impairment) at baseline. Exclusion Criteria: * Participants with a DSM-5 diagnosis of Other Specified or Unspecified Attention Deficit/Hyperactivity Disorder. * Participants has a current comorbid psychiatric disorder that either could be expected to require treatment with medications prohibited in this trial, or to confound efficacy or safety assessments. Examples include, but are not limited to, psychotic disorder, bipolar disorder, generalized anxiety disorder, obsessive-compulsive disorder, panic disorder, a current major depressive episode, or posttraumatic stress disorder, as established by the Mini International Neuropsychiatric Interview (MINI). * In the opinion of the investigator, participants has not derived significant therapeutic benefit from 2 or more ADHD therapies of 2 different classes (eg, amphetamine and methylphenidate) given with an acceptable dose and duration during adulthood (aged 18 or older). NOTE: If participants has not derived significant therapeutic benefit due to an inability to tolerate side effects, eligibility can be discussed on case-by-case basis with the medical monitor. * Participants who have a positive alcohol test (via breathalyzer or blood), a positive drug screen assessed prior to the baseline visit for cocaine, other illicit drugs (including marijuana), or prescription or over-the-counter (OTC) ADHD medications will be early terminated. This includes medications such as opioids or benzodiazepines taken without prescription. * In the opinion of the investigator, the participants is unable to adhere to the treatment regimen or other requirements outlined in the protocol.