The maximum dosage for a vitamin, more commonly referred to as the tolerable upper intake level, or UL, is the highest amount of daily intake of a particular vitamin that is considered safe. Each UL is set by the Food and Nutrition Board, which is a subgroup of the Institute of Medicine. Upper levels are designed to help prevent vitamin toxicities and are different for each vitamin.
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The upper limit for vitamin A is set at 3,000 micrograms per day. The risks of consuming excess amounts of the vitamin include liver problems, nausea, vomiting, headache, blurred vision, skin abnormalities and nervous system disorders. The Colorado State University Extension notes that it would be difficult to consume this amount of vitamin A through food alone, but it is important to watch vitamin A amounts in multivitamins.
Consuming excess amounts of vitamin D can result in hypercalcemia, which is an excess amount of calcium in the blood. Too much vitamin D can also cause nausea, vomiting, increased thirst, frequent urination and kidney problems. The upper limit for vitamin D is 50 micrograms per day.
Large amounts of vitamin E present the most risk for those who are on blood-thinning medications because it can result in an increased risk of blood hemorrhaging, according to the University of Nebraska Institute of Natural and Agricultural Resources. The upper limit is set at 1,000 milligrams per day, and it is noted that excess amounts usually come from supplements and fortified foods.
The food and nutrition board set the upper limit for vitamin C at 2,000 milligrams per day. Vitamin C is a water-soluble vitamin, which means it is not stored in the body. However, if you take too much at one time you can experience troubling symptoms. Excess amounts of the vitamin can cause diarrhea, bloating, kidney stones, iron overload, reduced availability of vitamin B-12 and loss of tooth enamel.
Eight B vitamins make up the vitamin B-complex group. These vitamins include thiamin, riboflavin, niacin, vitamin B-6, folate, vitamin B-12, biotin and pantothenic acid. Most of the B vitamins do not cause any adverse effects in high amounts, and therefore, these vitamins do not have upper limits. The vitamins that do have upper limits are niacin, vitamin B-6 and folate. The upper limit for niacin is 35 milligrams. Excess amounts can cause gastrointestinal problems and redness of the face and neck, which is called flushing. The upper limit for vitamin B-6 is set at 100 milligrams per day. Intake over this amount can cause damage to the nerves, which may result in abnormal sensations, such as numbness and tingling. Excess folate can also cause damage to the nerves. The upper limit is set at 1,000 micrograms per day.