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.
