Here is one reference, others I've seen are similar: https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/ https://ods.od.nih.gov/factsheets/VitaminD-

Here is one reference, others I've seen are similar: https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/ https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessiona... There is not complete consensus, but a lot of the large deltas are due to Vitamin D being fat soluble - if you are deficient, you can take far higher doses for a while and they will be stored in your liver. But if you continue to do so once you are at good levels, it can do damage: "Hypercalcemia, in turn, can lead to nausea, vomiting, muscle weakness, neuropsychiatric disturbances, pain, loss of appetite, dehydration, polyuria, excessive thirst, and kidney stones." That paper's recommendations are 600IU RDA, and that it is safe to take up to 4000IU without supervision of a doctor (i.e. regular blood tests to make sure you're not overshooting recommended blood concentration). This paper suggests up to 10000IU is safe: https://www.healthline.com/nutrition/how-much-vitamin-d-is-too-much#TOC_TITLE_HDR_4 https://www.healthline.com/nutrition/how-much-vitamin-d-is-t... and that "A daily intake ranging from 40,000–100,000 IU (1000–2500 micrograms), for one to several months, has been shown to cause toxicity in humans" (with several case reports). Apparently Dr. Fauci takes 6000IU per day. https://vitamindwiki.com/Dr.+Fauci+takes+6%2C000+IU+of+Vitamin+D+daily+%E2%80%93+Sept+2020 https://vitamindwiki.com/Dr.+Fauci+takes+6%2C000+IU+of+Vitam... While these are different, 4000IU, 6000IU, and 10000IU are well within an order of magnitude as upper limits of what you can tolerate without side effects ("upper limits" being more than RDA, but won't cause a separate medical problem) I've seen some studies say that 5000IU or 10000IU is safe, differ a little bit about the normal RDA, but there's a ton of similar ranges.