Vitamin D plays an important role in bones, muscles, immunity, and calcium regulation.
Vitamin D toxicity is unusual from food
or sunlight. When it occurs, it is usually the result of taking **very large amounts of vitamin D supplements for an extended period**.
Excessive vitamin D can cause dangerously high calcium levels.
Possible symptoms include:
* nausea
* vomiting
* weakness
* confusion
* excessive thirst
* frequent urination
* dehydration
Very high vitamin D levels may lead to **kidney stones and, in severe cases, kidney failure**.
People taking high-dose vitamin D for a documented deficiency should follow the dose and monitoring plan recommended by their healthcare professional.
## 3. Vitamin C
Vitamin C is water-soluble, so excess
amounts are largely eliminated through urine.
That does not mean extremely high doses are harmless.
Large supplemental doses may increase the amount of oxalate in the urine. In susceptible people, this can contribute to the formation of **calcium oxalate kidney stones**.
People who already have:
* kidney disease
* a history of kidney stones
* disorders that increase oxalate absorption
should be particularly cautious with high-dose vitamin C supplements.
Getting vitamin C from ordinary foods such as citrus fruits, berries, peppers, and vegetables is generally not the concern. The issue is primarily concentrated
supplements taken in much larger amounts than the body requires.
## 4. Niacin — Vitamin B3
Niacin is an essential B vitamin, but high doses of nicotinic acid have also been used medically to influence cholesterol levels.
At nutritional doses, niacin is not normally associated with liver damage.
At pharmacological doses, however, it can cause elevations in liver enzymes and, in some cases, clinically significant liver injury.
Certain sustained- or extended-release formulations have been particularly associated with liver toxicity when used improperly.
Possible warning signs can include:
* unusual fatigue
* persistent nausea
* pain
in the upper abdomen
* dark urine
* yellowing of the skin or eyes
High-dose niacin should not be treated like an ordinary daily vitamin.
## 5. Iron Supplements
Iron is essential for producing hemoglobin and carrying oxygen through the body.
Iron supplements can be very useful when someone has confirmed iron deficiency, but taking iron when it is not needed can be harmful.
Unlike some nutrients, the body has limited ability to eliminate excess iron.
Large amounts can accumulate and potentially damage organs, including the **liver**. NIH guidance notes that excess iron
may damage the liver and other organs.
This is why iron should generally not be taken long-term simply because someone feels tired.
Fatigue has many causes, and blood tests can help determine whether iron deficiency is actually present.
### Warning:
Accidental iron overdose can be particularly dangerous in children, so iron supplements should always be stored securely.
## 6. Green Tea Extract
