What are the side effects of niacin overdose?

daughter in law took niacin (500mg) and had a serious reaction, alergic with red, itch, tongue swelling, throat swelling, severe headache. what happened?

500mg is a very high dose, why did she do that? Very high doses of niacin have been shown to have adverse effects on the liver; these effects are reversible when the supplementation is stopped.
Usually an adverse reaction to niacin is a temporary flush which makes you turn red and perhaps slightly itchy, so that sounds like what your daughter had.
500mg a day is a therapeutic dose for some conditions, but should only be taken under professional supervision. If she has a medical condition that requires B3 supplementation, she should be being taken care of by a doctor anyway.
If she doesn't want to see a doctor, just tell her to cut down the dosage. 100-200mg per day may be okay. If it's not, her body will tell her quickly, as she has already found out.

Extemely high doses of niacin can cause niacin maculopathy, a thickening of the macula and retina which leads to blurred vision and blindness. The recommended daily allowance of niacin is 2-12 mg a day for children, 14 mg a day for women, 16 mg a day for men, and 18 mg a day for pregnant or breast-feeding women. Over 20 mg per day may produce hot flushes lasting 15-30 minutes in duration.it can also cause liver failure

Severe lack of niacin causes the deficiency disease pellagra, wherein a mild deficiency slows down the metabolism decreasing cold tolerance. Extemely high doses of niacin can cause niacin maculopathy, a thickening of the macula and retina which leads to blurred vision and blindness. [3] The recommended daily allowance of niacin is 2-12 mg a day for children, 14 mg a day for women, 16 mg a day for men, and 18 mg a day for pregnant or breast-feeding women.[4] Over 20 mg per day may produce hot flushes lasting 15-30 minutes in duration.
The liver can synthesize niacin from the essential amino acid tryptophan (see below), but the synthesis is extremely slow; 60 mg of tryptophan are required to make one milligram of niacin.[citation needed] Dietary niacin deficiency tends to occur only in areas where people eat corn, the only grain low in niacin, as a staple food, and that don't use lime during maize (corn) meal/flour production. Alkali lime releases the tryptophan from the corn so that it can be absorbed in the gut, and converted to niacin.[5]


[edit] Biosynthesis
The 5-membered aromatic heterocycle of the essential amino acid, tryptophan, is cleaved and rearranged with the alpha amino group of tryptophan into the 6-membered aromatic heterocycle of niacin by the following reaction:


Biosynthesis: Tryptophan 鈫?kynurenine 鈫?niacin


[edit] Food Sources
Animal products: Fruits and vegetables: Seeds: Fungi:
liver, heart and kidney
chicken
fish: tuna, salmon
milk
eggs
leaf vegetables
broccoli
tomatoes
carrots
dates
sweet potatoes
asparagus
avocados
nuts
whole grain products
legumes
saltbush seeds
mushrooms
brewer's yeast



[edit] Other uses
Niacin plays an important role in the production of several sex and stress-related hormones, particularly those made by the adrenal gland. It also plays a role in removing toxic and harmful chemicals from the body.[5] There is evidence that doses of 500-1000 mg can terminate a bad trip on LSD, a synthetic indole, or enhance the MDMA experience. [citation needed]

Niacin, when taken in large doses, increases the level of high density lipoprotein (HDL) or "good" cholesterol in blood, and is sometimes prescribed for patients with low HDL, and at high risk of heart attack.[6] Niacin (but not niacinamide) is also used in the treatment of hyperlipidemia because it reduces very low density lipoprotein (VLDL), a precursor of low density lipoprotein (LDL) or "bad" cholesterol, secretion from the liver, and inhibits cholesterol synthesis.[7]

Niacin in itself is not toxic, but the chemicals converted by niacin are toxic to the skin and liver in overdose, and high doses of niacin should only be reached with gradual increase. Studies in laboratory animals have demonstrated behavioral changes when large doses of niacin are given.[8]

Anyone who takes that much niacin will have a reaction like that. It is no more an allergy than falling asleep after taking a sleeping pill is an allergy.

I don't know why she took it, but niacinamide (check that spelling) will not cause the flushing, even in large doses.
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she is obviously trying to flush something out of her system. its funny because when i took it, it turned me red and i was so hot i layed on icepacks, no one mentioned that..guess im different