Niacin is sometimes used in high doses for cholesterol-related reasons, but it should not be treated like an ordinary daily vitamin at those levels. The NIH notes that high doses of nicotinic acid taken over months or years can be hepatotoxic, meaning harmful to the liver, and very high intakes can cause liver damage.
Niacin from food is not the same risk as high-dose pills. The danger usually comes from large supplemental doses, especially extended-release or sustained-release forms used without medical supervision.
4. Vitamin C in Very High Doses
Vitamin C is water-soluble and important for immune function and collagen production, but very large doses are not harmless for everyone. The NIH notes that high vitamin C intake may increase urinary oxalate and uric acid excretion, and this can contribute to kidney stone risk in some people.
A large cohort study found supplemental vitamin C intake was associated with a higher risk of kidney stones in men. People with a history of kidney stones, chronic kidney disease, or oxalate problems should be extra cautious with high-dose vitamin C powders or tablets.
5. Iron Supplements
