Inclusion Criteria:  Male and female outpatients ≥ 18 and ≤ 65 years of age. Patients with the diagnosis of Diagnostic and Statistical Manual of Mental Disorders-5 (DSM-5) ADHD, as determined by clinical evaluation and confirmed by structured interview (MINI). Adequate organ function. Able to understand the study procedures and understand risks associated with the study, and sign written informed consent to participate in the study. Must be willing to refrain from more than 6 standard alcoholic drinks a week, more than 10 cigarettes a day, and more than 2 cups of coffee a day throughout the study treatment period (6 weeks) and until the final dose.  Exclusion Criteria:  Past or present diagnosis of a primary psychotic disorder or first degree relative with a psychotic disorder. Past or present bipolar disorder (DSM-5). Any lifetime history of suicide attempt. Not willing or able to stop any prescription or non-prescription ADHD medications. Use of investigational medication/treatment in the past 30 days. Patients with a positive urine drug screen. Pregnant or nursing females. Non-German (Switzerland site) or non-Dutch (Netherlands site) speaking patients.