Inclusion Criteria:  The participant has provided signed informed consent. Males and/or females aged 18-70 (extremes included). Participants with a primary diagnosis of ADHD according to DSM-5 (314.01) criteria (diagnosis to be made using the Mini-International Neuropsychiatric Interview (MINI) 7.0.2 and confirmed by the Diagnostic Interview for ADHD in Adults (DIVA 5.0). Participants with a comorbid anxiety and depressive disorder will be permitted, as long as ADHD is judged to be the primary diagnosis. Participants who score an ASRS of ≥ 4 in Part A at both Screening and Baseline, representing non-response to current stable psychostimulant treatment Participants are on a stable dose (> 4 weeks) of their existing long-acting psychostimulant (any type) prior to entry into the study. Participants are on a stable dose of any other psychotropic medication (> 8 weeks) to treat comorbid conditions, except antipsychotics.  On the basis of a physical examination, medical history and basic laboratory screening, the patient is, in the investigator's opinion, in a suitable condition.  Basic laboratory screening includes:  Chemistry: Electrolytes, ALT, Albumin, Alkaline Phosphatase, AST, Bilirubin Total protein, Creatinine, Urea (BUN), CK, GGT, Potassium, Sodium, Calcium, Glucose (Fasting), Bilirubin Direct, Bicarbonate, Chloride, Urate (for Uric Acid), LD, Magnesium, Phosphorus, Amylase CBC: Hematocrit, Hemoglobin, RBC, WBC + differential, abs. Platelet Count Drug Screen (urine-8 tests): amphetamines, benzodiazepines, barbiturates, methadone, cocaine, opiates, cannabinoids, PCP Standard Urinalysis Lipid Assessment: Cholesterol, HDL, LDL-calc, Triglycerides Prolactin  Willing and able to attend study appointments in the correct time windows.  Exclusion Criteria:  Participants meeting one or more of the following criteria cannot be selected for inclusion:  Any other primary mental health disorder in the previous six months. Alcohol or drug abuse as defined in the DSM-5 criteria within the last six months. Mania, hypomania as defined in the DSM-5 criteria. Any psychotic disorder. Eating disorders as defined in the DSM-5 criteria. Any cognitive disorder or dementia within 3 months before the baseline visit. A history of Seizure Disorder (Epilepsy or other). Clinical interpretation of apparent suicide risk. Commencement of formal psychotherapy for 4 weeks prior to entry into the study and/or during the course of the study. Existing treatment with any antipsychotic as mono- or adjunct therapy at the time of the study. Change in use of medications. Laboratory values at screening or in medical history that may be considered through clinical interpretation to be significant, including positive drug and alcohol tests. Diseases that could through clinical interpretation interfere with the assessments of safety, tolerability and efficacy of study treatment. Serious illness: liver or renal insufficiency, cardiac, vascular, pulmonary, gastrointestinal, endocrine, neurological, infectious, neoplastic or metabolic disturbance. If female, the subject is pregnant or lactating or intending to become pregnant before, during, or within 30 days after participating in this study, or intending to donate ova during such time period. The participant has received electroconvulsive therapy, vagal nerve stimulation, or repetitive transcranial magnetic stimulation within 6 months prior to Screening. The participant is, in the opinion of the investigator, unlikely to be able to comply with the clinical trial protocol or is unsuitable for any other reasons. Contraindications and Warning Precautions as per the U.S. Product Monograph will be followed.  Participants must discontinue the use of recreational drugs including cannabis for at least 2 weeks prior to entry into the study. Participants must limit alcohol intake to a maximum of 3 standard drinks per week during the study period.