This does not mean everyone should fear supplements. Many people use them safely when they have a real deficiency or a doctor’s recommendation. The problem starts when people take large amounts, combine several products, follow viral advice online, or use supplements instead of proper medical care.
The FDA explains that dietary supplements are regulated differently from regular foods and drugs, and products marketed to diagnose, treat, cure, or prevent disease are legally considered drugs, even if they are sold as supplements.
Below are 9 vitamins and supplements that deserve extra caution because they may affect the liver, kidneys, or both when misused.
1. Vitamin A
Vitamin A is important for vision, immunity, and skin health, but too much preformed vitamin A from supplements can build up in the body because it is fat-soluble. The NIH Office of Dietary Supplements lists 3,000 mcg RAE as the adult tolerable upper intake level for preformed vitamin A, and excessive intake can lead to toxicity.
High intake over time may affect the liver and cause symptoms such as headache, dizziness, nausea, skin changes, bone pain, or more serious complications. People should be especially careful with high-dose vitamin A capsules, “skin” supplements, and multivitamins that already contain vitamin A.
2. Vitamin D
Vitamin D supports bones, muscles, and immune function, but high-dose vitamin D can become toxic. Vitamin D toxicity usually happens from excessive supplementation, not from normal sunlight exposure or food. It can lead to high calcium levels in the blood, which may cause nausea, weakness, confusion, dehydration, frequent urination, kidney stones, or kidney damage.
Vitamin D should ideally be guided by a blood test, especially if someone is taking strong doses, has kidney disease, has high calcium levels, or is also using calcium supplements.
3. Niacin, Also Called Vitamin B3
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
