Vitamins >> Vitamin B3 Safety

Toxicity

Niacin from foods is not known to cause adverse effects. Although one study noted adverse effects from the consumption of bagels to which were added 60 times the normal amount of niacin fortification, most adverse effects have been reported with pharmacologic preparations of niacin.

Nicotinic acid: Flushing, itching and gastrointestinal disturbances such as nausea and vomiting are common. Hepatotoxicity (liver cell damage), including elevated liver enzymes and jaundice, has been observed at intakes as low as 750 mg of nicotinic acid/day for less than 3 months. Hepatitis has been observed with timed-release nicotinic acid on as little as 500 mg/day for 2 months, although almost all reports of severe hepatitis have been associated with the timed-release form of nicotinic acid at doses of 3 to 9 grams per day used to treat high cholesterol for months or years. Immediate-release (crystalline) nicotinic acid appears to be less toxic to the liver than extended release forms. Immediate-release nicotinic acid is often used at higher doses than timed-release forms, and severe liver toxicity has occurred in individuals who substituted timed-release niacin for immediate-release niacin at equivalent doses. Skin rashes and dry skin have been noted with nicotinic acid supplementation. Transient episodes of low blood pressure (hypotension) and headache have also been reported. Large doses of nicotinic acid have been observed to impair glucose tolerance, likely due to decreased insulin sensitivity. Impaired glucose-tolerance in susceptible (pre-diabetic) individuals could result in elevated blood glucose levels and clinical diabetes. Elevated blood levels of uric acid, occasionally resulting in attacks of gout in susceptible individuals, have also been observed with high-dose nicotinic acid therapy. Nicotinic acid at doses of 1.5 to 5 grams/day has resulted in a few case reports of blurred vision, and other eye problems, which have generally been reversible upon discontinuation. People with abnormal liver function or a history of liver disease, diabetes, active peptic ulcer disease, gout, cardiac arrhythmias, inflammatory bowel disease, migraine headaches, and alcoholism may be more susceptible to the adverse effects of excess nicotinic acid intake than the general population.

Nicotinamide: Nicotinamide is generally better tolerated than nicotinic acid. It does not generally cause flushing. However, nausea, vomiting, and signs of liver toxicity (elevated liver enzymes, jaundice) have been observed at doses of 3 grams/day. Nicotinamide has resulted in decreased insulin sensitivity at doses of 2 grams/day in adults at high risk of insulin-dependent diabetes.

The tolerable upper intake level (UL): Flushing of the skin primarily on the face, arms, and chest is a common side effect of nicotinic acid and may occur initially at doses as low as 30 mg/day. Although flushing on nicotinamide is rare, the Food and Nutrition Board set the tolerable upper intake level (UL) for niacin (nicotinic acid and nicotinamide) at 35 mg/day to avoid the adverse effect of flushing in the general population. The UL is not meant to apply to individuals who are being treated with a nutrient under medical supervision, as should be the case with high-dose nicotinic acid for elevated blood cholesterol levels.

*Source of intake should be from food and formula only.

Drug interactions

Coadministration of nicotinic acid with lovastatin (another cholesterol lowering medication) may have resulted in rhabdomyolysis in a small number of case reports. Rhabdomyolysis is a relatively uncommon condition in which muscle cells are broken down, releasing muscle enzymes and electrolytes into the blood, sometimes resulting in kidney failure. A 3-year randomized controlled trial in 160 patients with documented coronary heart disease (CHD) and low HDL levels found that a combination of simvastatin (Zocor) and niacin increased HDL2 levels, inhibited the progression of coronary artery stenosis (narrowing), and decreased the frequency of cardiovascular events, such as myocardial infarction and stroke. However, concurrent therapy with antioxidants (1000 mg/d vitamin C, 800 IU/d alpha-tocopherol, 100 mcg/d of selenium, and 25 mg/d beta-carotene) diminished the protective effects of the simvastatin-niacin combination. Although the mechanism for these effects is not known, some scientists have questioned the benefit of concurrent antioxidant therapy in patients on lipid lowering agents.

Sulfinpyrazone is a medication for the treatment of gout that promotes excretion of uric acid from the blood into urine. Nicotinic acid may inhibit this "uricosuric" effect of sulfinpyrazone. Long-term administration of the cancer chemotherapy agent, 5-Fluorouracil (5-FU), has been reported to cause symptoms of pellagra. Niacin supplementation is recommended during long-term treatment of tuberculosis with isoniazid. Isoniazid is a niacin antagonist and long-term treatment has resulted in pellagra-like symptoms. Estrogen and estrogen-containing oral contraceptives increase the efficiency of niacin synthesis from tryptophan, resulting in a decreased dietary requirement for niacin .

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