Study of the Safety and Efficacy of OPC-34712 as a Complementary Therapy in the Treatment of Adult Attention Deficit/Hyperactivity Disorder

Inclusion Criteria: * Male and female outpatients 18 to 55 years of age, inclusive, at the time of informed consent. * Participants with a primary diagnostic and statistical manual of mental disorders, fourth edition, text revision (DSM-IV-TR) diagnosis of ADHD (including inattentive, hyperactive, and combined subtypes) as confirmed by the Conners' Adult ADHD diagnostic interview (CAADID). Participants may have received prior treatment for adult ADHD, may be currently receiving treatment for adult ADHD at screening, or may be treatment-naive. * Participants willing to discontinue all prohibited psychotropic medication starting from the time of signing the informed consent and during the study period. Exclusion Criteria: * Females who are breast-feeding and/or who have a positive pregnancy test result prior to receiving study drug. * Participants with an adequate response, as determined by the investigator, to any stimulant taken for the treatment of adult ADHD after 18 years of age. * Participants with a clinically significant current Axis II (DSM-IV-TR) diagnosis of borderline, antisocial, paranoid, schizoid, schizotypal, or histrionic personality disorder. * Participants who participated in a clinical trial within the last 180 days or who participated in more than two clinical trials within the past year.