Hypervitaminosis is vitamin toxicity — illness caused by excessively high intakes of vitamins, well beyond nutritional need. The fat-soluble vitamins are the culprits: vitamin A excess causes liver damage, raised intracranial pressure, and birth defects; vitamin D excess drives blood calcium dangerously high, calcifying kidneys and vessels; vitamin E and K megadoses disturb clotting. Water-soluble vitamins are generally safer — excess is excreted — though B6 megadoses damage nerves and niacin flushes and harms the liver. Hypervitaminosis is almost never dietary; it is a supplement disease, born of the “more must be better” fallacy. The tolerable upper intake levels exist precisely to mark where benefit ends and poisoning begins.