Vitamin D as a Therapeutic Adjunct in the Stimulant Treatment of ADHD: a Proof-of-concept Tele-health Study of Stimulant-induced Improvement in Neurocognitive Functioning.

Inclusion Criteria: * Age 18-50 years * Voluntary, written, informed consent * Physically healthy by medical and psychiatric history * DSM-5 diagnosis of ADHD * Point of Care Test results for Vitamin D equal or higher than 20 ng/ml * English speaking Exclusion Criteria: * Medical contraindication to calcitriol administration (e.g., history of hypersensitivity to calcitriol or any component of the formulation, hypercalcemia or vitamin D toxicity) * History of substance dependence (e.g., alcohol, opiates, sedative hypnotics), except for nicotine * A primary major DSM-V psychiatric disorder (e.g., schizophrenia, bipolar disorder, major depression, etc.) as determined by the Structured Clinical Interview for DSM-V (SCID), except ADHD * A history of significant medical (e.g., cardiovascular, diabetic/metabolic) or neurological (e.g., cerebrovascular accidents, seizure, traumatic brain injury) illness * Current use of psychotropic and/or potentially psychoactive prescription medications, except prescribed stimulants * Use of any prescription medications and/or over-the-counter medications, vitamins (including vitamin D) and/or herbal supplements which could have a negative clinical interaction with calcitriol or which could confound scientific results of the study, within 2 weeks prior to each test day (e.g., thiazide diuretics, Mg based antiacids, digoxin, etc,). * Levels of 25(OH)D3 below 20 ng/ml . * History of kidney stones within the past 5 years * History of renal failure * History of parathyroid disorder (hyper or hypoparathyroidism) * History of osteoporosis or any pathologic fractures * Vitamin D supplementation in any form in the past 3 months * Known hypersensitivity to calcitriol * Malabsorption syndromes (i.e. Celiac sprue)