Pharmacogenomics of Stimulant Treatment Response

Inclusion Criteria: Patients will be eligible for participation if all the following are true. * Aged 6 - 24 years. * Located in Western Canada (i.e., Alberta, British Columbia, Saskatchewan, Manitoba). * Primary diagnosis of ADHD (all types). * Starting Methylphenidate (excluding immediate release forms) treatment. Exclusion Criteria: Patients will be excluded from participation if any of the following are true. * Co-occurring psychotic, bipolar or eating disorders. * Significant risk of suicide. * An intellectual disability, or diagnosis of autism spectrum disorder (ASD) or tics/Tourette disorders. * Past 12-month high-risk alcohol or substance use defined as monthly or more frequent use. * Psychotherapy or brain stimulation-based therapy initiated within 8 weeks of referral or plans to initiate/change these types of therapies during the study * History of liver or bone marrow (hematopoietic cell) transplant as these events can result in ambiguous genomic results.