Following Yale University resuming in-person research activities, the investigators no longer had dedicated funding for the study and were unable to secure additional funding to meet the research goals.

The purpose of this study is to assess vitamin D as a therapeutic adjunct in the stimulant treatment of ADHD.

The specific aim of this study is to determine whether acute calcitriol (vitaminD) administration (compared to placebo) enhances the neurocognitive effects of of current stimulant medications as measured by tasks of vigilance/attention, spatial working memory, and reversal learning in individuals with (attention deficit hyperactivity disorder (ADHD) using a randomized, double-blind, placebo controlled, within-subject, two-day study design.

Primary Hypothesis: It is hypothesized that calcitriol (versus placebo) administration will enhance positive neurocognitive effects of current stimulant medications in individuals with ADHD.

Secondary (Exploratory) Hypothesis: It is hypothesized that calcitriol (versus placebo) administration alone will also enhance neurocognitive performance on tasks of attention/vigilance and/or spatial working memory.

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)