Niacin (vitamin B3)

Is niacin safe?

At the 14 to 16 mg a day that adults need, and the amounts in a typical multivitamin, yes. The safety problems start with stand-alone niacin products, which in the NIH Dietary Supplement Label Database typically hold 100 to 500 mg per tablet, many of them as nicotinic acid. At those doses niacin acts as a drug: it changes blood fats, and it brings drug side effects with it, from flushing to liver injuryDrug-induced liver injuryLiver damage caused by a medicine, herbal product or supplement. It is usually rare and unpredictable, but it can be serious.More in the glossary and higher blood sugar.

That gap is why niacin scores low. The US upper limit is 35 mg a day; a single 500 mg tablet is 14 times that.

Why does niacin make you flush?

Nicotinic acid triggers the skin to release prostaglandin D2, which widens blood vessels. The result is redness, heat and itching over the face, arms and chest, starting at doses as low as 30 mg. The US upper limit of 35 mg was set to prevent exactly this. EFSA's limit for nicotinic acid is lower still, at 10 mg a day.

Flushing is unpleasant but rarely dangerous, and it is the main reason people stop. Niacinamide (nicotinamide), the form used in most multivitamins and fortified foods, rarely causes it, which is why EFSA allows up to 900 mg a day of that form.

Recalls show that this distinction gets lost in factories. In 2012 a liquid supplement was recalled after nicotinic acid was used instead of the intended form, and in 2024 a B-complex product was recalled because it contained nicotinic acid while the label said niacinamide.

Can niacin damage the liver?

Yes, and this is the serious risk. LiverToxLiverToxA free US National Institutes of Health database that summarises what is known about liver injury caused by medicines, herbs and supplements.More in the glossary, the US government database on drug-induced liver injuryDrug-induced liver injuryLiver damage caused by a medicine, herbal product or supplement. It is usually rare and unpredictable, but it can be serious.More in the glossary, says high doses and certain formulations of niacin have caused acute liver injury that can be severe and fatal.

The formulation matters. In a 1994 JAMA trial, 46 patients took increasing doses of either immediate-release or sustained-release niacin. Twelve of 23 on the sustained-release form (52%) developed liver toxicity. None on immediate-release did. The Linus Pauling Institute notes that liver injury has been seen at 750 mg a day of nicotinic acid and with extended-release forms at 500 mg a day for two months, and that severe cases followed patients switching from immediate-release to slow-release tablets at the same dose.

Warning signs are tiredness, nausea, loss of appetite, dark urine and yellow skin or eyes. Anyone taking high-dose niacin should have liver blood tests arranged by a doctor.

Does niacin prevent heart attacks?

For decades niacin was used to raise "good" HDL cholesterol. Two large trials tested whether that helped on top of a statin.

HPS2-THRIVE also found new harms: 1.3 percentage points more new diabetes diagnoses, plus more serious bleeding and infections. The European Medicines Agency suspended niacin-laropiprant in January 2013 after reviewing those data, concluding that the benefits no longer outweighed the risks. In 2016 the FDA withdrew the approved uses of extended-release niacin in combination with statins, at the request of the drug makers.

A 2024 study in Nature Medicine added a new question. Researchers found that heart patients with high blood levels of 4PY and 2PY, breakdown products of excess niacin, had about 1.6 to 2 times the risk of heart attack, stroke or death over three years. In mice, 4PY increased inflammation in blood vessel walls. This is an association in heart patients, not proof that supplements cause heart disease, but it points away from the idea that more niacin is better.

Drug interactions

High-dose niacin interacts with several common drugs:

• Statins and gemfibrozil: higher risk of muscle breakdown. Tell your doctor about muscle pain or weakness, especially with atorvastatin or simvastatin.
• Diabetes drugs: niacin raises blood sugar and can undo their effect.
• Gout drugs: niacin reduces uric acid excretion and opposes drugs such as probenecid.
• Blood pressure drugs: niacin itself can cause short drops in blood pressure, so dizziness is more likely.
• Paracetamol, amiodarone and carbamazepine: may add to liver risk.

An antioxidant mix including vitamin E blunted the benefit of simvastatin plus niacin in the three-year HATS trial.

Who should be careful

• People with liver disease, active stomach ulcers, gout or diabetes: talk to your doctor before taking any niacin above multivitamin doses.
• People on statins: do not add niacin for cholesterol without your doctor; it has not been shown to prevent heart attacks.
• Anyone taking red yeast rice or other cholesterol supplements: combining them stacks liver and muscle risks.
• Buyers: check the form on the label (nicotinic acid, niacinamide or inositol hexanicotinate), and never swap an immediate-release product for a slow-release one at the same dose.

How we scored it

Organ toxicity (35) is the weakest factor: LiverToxLiverToxA free US National Institutes of Health database that summarises what is known about liver injury caused by medicines, herbs and supplements.More in the glossary documents severe and sometimes fatal liver injuryDrug-induced liver injuryLiver damage caused by a medicine, herbal product or supplement. It is usually rare and unpredictable, but it can be serious.More in the glossary, and a randomised trial found liver toxicity in half the patients on sustained-release niacin. Dose safety (45) reflects common tablets far above the 35 mg upper limit. Interactions (55) and regulatory actions (60) follow the statin, diabetes and gout problems and the 2013 EU suspension. Quality (65) is held back by recalls where the wrong form of niacin went into the bottle.