Can you get too much niacin (B3)?

The Tolerable Upper Intake Level for niacin is 35 mg a day for adults aged 19 and over, set on the basis of flushing from nicotinic acid.

What this means for you

From food, no. The upper limit of 35 mg a day applies to supplements and it exists because of flushing rather than organ damage. Niacin in chicken, tuna, peanuts or fortified cereal has never been the problem.

The flush is worth describing, because people meeting it for the first time find it alarming. Twenty minutes or so after a dose of nicotinic acid, the face, neck and chest turn red and burn or itch. It fades within an hour or two and it does no harm, though it is the single commonest reason people stop taking it.

Not for everyone. High dose niacin, in the gram range used for cholesterol, is a different matter. At those doses it can raise liver enzymes, worsen blood sugar control and raise uric acid. In people who already have type 2 diabetes the blood sugar part has been measured. On long-term treatment fasting glucose rose by 0.085 mmol/L, which is small and was enough for the reviewers to say it needs watching. The same pooled trials raised HDL cholesterol by 0.27 mmol/L and lowered triglycerides by 0.39. That is prescription territory with blood tests, rather than something to run on your own from a supplement shop.

Myth. Nicotinic acid raises HDL cholesterol, which made it look like a heart drug for decades. Across 23 randomised trials in 39,195 people it did not reduce deaths, heart attacks or strokes, and a meta-analysis of 35,760 patients with existing heart disease found no protective effect. The HDL number moved and the outcomes did not.

The one thing a high dose has actually been shown to do

Established. In 386 people who had already had at least two non-melanoma skin cancers, 500 mg of nicotinamide twice a day cut new ones by 23 per cent over a year. Squamous-cell cancers fell by 30 per cent and the rough sun-damage patches called actinic keratoses by 13 per cent. Two things keep this from being general advice. The benefit stopped when the tablets stopped, and everyone in the trial had a history of skin cancer already. A gram a day is close to thirty times the 35 mg ceiling, so this belongs to a dermatologist who knows your skin.

One honest caveat about the limit itself. Pooling 47 studies in 11,741 people, serious side effects in otherwise healthy adults started below 1,000 mg a day, which is a long way above the 35 mg ceiling. The authors of that analysis argued the current limits are conservative. Conservative is a reasonable place for a ceiling to sit, and it also means a 50 mg B-complex tablet is not a medical emergency.

What the 35 mg ceiling actually is

It helps to know what that number was built on, because it is not what most people assume.

The American upper limit of 35 mg is set on skin flushing rather than on organ damage. It applies to supplemental niacin, not to food. And it covers nicotinamide as well as nicotinic acid, even though nicotinamide does not cause the flush the limit was derived from. It also does not apply to people taking niacin under medical supervision, which is most of the people on a large dose.

Not for everyone. Europe answers the same question with two different numbers, and this is the single most useful thing on this page. The EU upper limit is 10 mg a day for nicotinic acid and 900 mg a day for nicotinamide. Same vitamin, ninetyfold difference, because the two forms behave differently in the body. EFSA separately judged nicotinamide riboside chloride safe up to 300 mg a day for healthy adults and 230 mg in pregnancy and breastfeeding. A niacin supplement label is unreadable until you find which molecule is in it, and the word niacin on the front does not tell you.

Across fourteen reviews of niacin side effects, nicotinamide came out easier on the body than nicotinic acid. The problems recorded were stomach upset, liver dysfunction, flushing, rash and fatigue. They depended on the dose, and they depended heavily on what else was wrong with the person, with kidney dysfunction the clearest example. Most of them resolved when the dose came down.

Established. None of this touches food. No study has ever documented harm from the niacin naturally present in food. Every reported adverse effect comes from a supplement, a fortificant or a medicine.

One finding that does point at food, and is not settled

Unsettled. A 2024 study tracked 2PY and 4PY, the terminal breakdown products of excess niacin, and found they predicted heart attacks and strokes over three years. The hazard ratios ran from 1.64 to 2.02 for 2PY and from 1.89 to 1.99 for 4PY across two validation cohorts. The part that makes it awkward is who was left out: the researchers deliberately excluded everyone taking niacin as a drug, so what the finding is pointing at is ordinary niacin from food and from fortified flour. Fortification is one of public health's long-running successes, metabolites are not outcomes, and one cohort study does not overturn a record like that. It is not a reason to avoid bread. It is a reason to know the question of how much niacin a whole population should get has been reopened, and that you will hear more about it.

Written from the evidence cards and the data on this page, then checked back against the sources. Bioma Learn has no human medical reviewer at this time and we say so rather than invent one. Information, not medical advice. How we verify.

Upper limits apply to everything you take in, from supplements and, for most nutrients, from food as well. Reaching one from whole food alone is uncommon. Supplements are where people cross the line, usually without meaning to.

Full guide: Niacin: the vitamin that became a heart drug and then failed the trials