"Like the other steroid hormones, Vitamin D acts on multiple organs and only tiny amounts of the molecule are required to have substantial effects. Anyone who has gone through puberty understands the impressive physiologic changes brought about by relatively small shifts in hormone levels. Not surprisingly, most steroid hormones are considered medications that require a physician’s order. Vitamin D is an exception. In the U.S., high dose Vitamin D can be purchased by anyone, without a prescription, in most drugstores and grocery stores. It is called a dietary supplement, which sounds safer than a medication, but this is misleading. Due to its role in calcium absorption, high dose Vitamin D can cause serious problems."
https://www.devaboone.com/post/vitamin-d-part-2-shannon-s-story