Vitamins are essential for our health, but it's important to remember that you only need them in small amounts. While it's nearly impossible to get too much of any vitamin from food, taking large doses of supplements over long periods of time can lead to an overdose. Vitamin overdose occurs when a person ingests much more than is recommended daily, over an extended period of time. Certain water-soluble vitamins have no observable toxicity and therefore do not have an established UL.
These vitamins include vitamin B1 (thiamine), vitamin B2 (riboflavin), vitamin B5 (pantothenic acid), vitamin B7 (biotin), and vitamin B12 (cobalamin). The body can excrete excessive amounts of water-soluble vitamins, such as vitamin C, but it can retain fat-soluble vitamins such as vitamin A, which can be toxic. Taking too much vitamin A, D, or E can cause potentially harmful side effects. Vitamin D is made by the body after skin is exposed to sunlight, and excessive and prolonged consumption of vitamin B6 can cause serious neurological symptoms, skin lesions, sensitivity to light and nausea.
Taking large amounts of vitamin C is not life-threatening, but it can cause diarrhea, nausea and abdominal cramps and has been linked to kidney stones. Choline is a B-complex vitamin that the body needs for several biological processes and needs it to produce a brain chemical called acetylcholine. Beta-carotene is the most important provitamin A (found mainly in vegetables) and is converted to vitamin A as needed by the body. Vitamin A supplements may allow you to delay the use of reading glasses because they promote the function of the human eye, but a serious overdose of vitamin A can actually cause death. It's important to remember that while water-soluble vitamins are not easily stored in body tissues and can be easily excreted, fat-soluble vitamins are more easily retained and accumulated in the body and are therefore easier to overdose on them.
Therefore, it's important to be aware of the recommended daily intake levels for each type of vitamin and not exceed them.