Clinical Research Directory

Inclusion Criteria: * Participants diagnosed primarily with ADHD (excluding other specified or unspecified attention-deficit/hyperactivity disorder) according to DSM-5 criteria, based on information collected through interviews using the Japanese version of the Conners' Adult ADHD Diagnostic Interview for DSM-IVTM (CAADID). * Participants with an AISRS total score meeting the following criteria: * Not receiving medication treatment for ADHD: 28 points or higher * Receiving medication treatment for ADHD: 22 points or higher Exclusion Criteria: * Participants who are pregnant or breastfeeding, and those who test positive for pregnancy at screening. * Participants have 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 (current or lifetime), bipolar disorder (current or lifetime), generalized anxiety disorder, obsessive-compulsive disorder, panic disorder, a current major depressive episode, or posttraumatic stress disorder, as established by the MINI. * Participants diagnosed with a personality disorder according to DSM-5 criteria. * Participants diagnosed with autism spectrum disorder according to DSM-5 criteria. * Participants diagnosed with intellectual disability and an IQ score below 70. * Participants who, in the judgment of the principal or sub-investigator, or based on the following criteria, are at significant risk of suicide: • Clear suicidal ideation or a history of suicidal behavior (within the past 6 months) as indicated by a ""yes"" answer to questions 4 or 5 in the suicidal ideation section of the C-SSRS Baseline/Screening version at screening. * Participants who have a current or past episode of substance-related disorders (excluding tobacco-related disorders) according to DSM-5 diagnostic criteria. * Participants diagnosed with eating disorders and feeding disorders according to DSM-5 diagnostic criteria. * Participants who show a positive reaction in alcohol tests or urine drug tests at screening. * Participants with complications or a history of the following neurological disorders: * Epilepsy * Seizures (other than infantile febrile seizures) * Syncope * Tourette's disorder * History of significant head trauma with clinically significant loss of consciousness * Dementia * Cerebrovascular disease * Parkinson's disease * Intracranial lesions * Other severe neurological disorders * Participants with complications or a history of cardiovascular diseases. * Participants with clinical laboratory test results at screening that meet any of the following criteria: * Platelets \<= 75,000/mm3 * Hemoglobin \<= 9 g/dL * Neutrophils, absolute \<= 1000/mm3 * AST \> 2 x ULN * ALT \> 2 x ULN * Creatinine \>= 2 mg/dL * CPK \>= 2 x ULN (except for the cases that the medical monitor determined that participant's inclusion is possible based on the discussion about the participant's condition with the investigator or subinvestigator) •Abnormal values for both free T4 and TSH• * Participants who cannot agree to discontinuation of prohibited concomitant medication, such as ADHD medication or antidepressants.