Quick answer

Within the 35 mg upper limit for supplements, the main side effect of niacin is flushing from nicotinic acid: red, warm, tingling skin that usually passes. Side effects grow with the dose. At 1,000 mg a day or more, the range used as a medicine, they include liver harm, so those doses need a doctor.^1,2

Key facts

Flushing typically starts at 30 to 50 mg or more of nicotinic acid and is considered unpleasant rather than toxic.¹
Nicotinamide does not flush, but 500 mg a day or more can cause loose stools and easy bruising.²
Liver harm from nicotinic acid is more likely with extended-release forms and with high doses taken for months or years.¹
Up to 88% of people in trials of prescription niacin reported flushing.³
No harm has been reported from niacin that occurs naturally in food.¹

The niacin flush

Nicotinic acid at 30 to 50 mg or more often causes flushing: the skin of the face, arms and chest turns red and can burn, tingle and itch. It usually passes and is considered unpleasant rather than toxic.^1,2

Flushing can come with headache, rash, dizziness or a drop in blood pressure.^1,2

The redness comes from small blood vessels just under the skin widening. It can appear within 30 minutes of a dose, or build up over days or weeks of repeated doses.¹

Tolerance to flushing builds over several weeks of steady use, so flushing is most likely when someone starts niacin or raises the amount.^3,4

NIH gives three ways to make it milder: take nicotinic acid with food, raise the amount slowly, or give the body time to get used to it.¹

Which forms flush

Nicotinamide, also called niacinamide, is the other main form. Its slightly different structure means it does not cause flushing, and it has fewer side effects than nicotinic acid.^1,2

Inositol hexanicotinate is often labeled 'flush free'. It does not cause flushing, but on average about 30% less niacin is absorbed from it than from nicotinic acid or nicotinamide, which are almost completely absorbed.¹

If flushing bothers you, look for nicotinamide or niacinamide on the ingredient line rather than nicotinic acid.¹

Side effects at high doses

At 1,000 mg a day or more, nicotinic acid can cause low blood pressure that makes falls more likely, extreme tiredness, high blood sugar, nausea, heartburn, stomach pain, and blurred vision or fluid buildup in the eye.^1,2

High doses of nicotinic acid taken for months or years can harm the liver, from raised liver enzymes to hepatitis and liver failure. This is more likely with extended-release forms, and severe cases have followed switching from immediate-release to sustained-release niacin at the same dose.^1,3

Nicotinamide does not flush, but at 500 mg a day or more it can cause diarrhea and easy bruising and can increase bleeding from wounds. At 3,000 mg a day or more it can cause nausea, vomiting and liver damage.^1,2

In trials of prescription niacin, the most common side effects were flushing, diarrhea, nausea, vomiting, a worse cough and itching. Up to 88% of people reported flushing, and women reported side effects more often than men.³

The prescription label lists active liver disease, an active stomach ulcer and arterial bleeding among the reasons not to take prescription niacin. It also advises caution for people who drink substantial amounts of alcohol or have had liver disease.³

The upper limit

The upper limit for niacin from supplements and medicines is 35 mg a day for adults, including during pregnancy and breastfeeding. It is 30 mg for ages 14 to 18, 20 mg at 9 to 13, 15 mg at 4 to 8 and 10 mg at 1 to 3. It is set at the level linked to flushing, covers both nicotinic acid and nicotinamide, does not count niacin in food, and does not apply to people taking niacin under medical supervision.^1,2

In one national survey, about 10% of people who took supplements with niacin reached or passed the limit. Check every product you take, not just the one labeled niacin.¹

Medicines and conditions to ask about

Ask a pharmacist before combining niacin with a statin, a blood thinner, a blood pressure medicine or a diabetes medicine. The prescription niacin label and the labels of those medicines describe effects that add up, mostly at high doses. Statins, atorvastatin, metformin and blood thinners each have their own page.^3,5,6

Tell your doctor before starting niacin if you have gout, diabetes, a bleeding problem, a stomach ulcer or liver disease⁴
Tell your doctor if you take isoniazid or pyrazinamide for tuberculosis, which lower niacin levels²
If you take prescription niacin, check with your doctor before adding any supplement with niacin or nicotinamide³
Ask first if you are pregnant or breastfeeding

What's in ours

What’s in ours

Men's Multi: Niacin, 30 mg NE per serving, from the label

Questions people ask

How long does a niacin flush last?

Usually not long. For extended-release prescription niacin, the patient leaflet says flushing usually starts 2 to 4 hours after a dose and may last a few hours, and that it tends to ease after several weeks of use.³

Is it safe to take niacin every day?

Food supplies niacin every day, and the upper limit is a daily figure: up to 35 mg a day from supplements is the most adults can take with little chance of harm. Daily doses in the medicine range need a doctor's care, because liver harm becomes more likely the longer they are taken.¹

Do niacin side effects differ for women?

In trials of prescription niacin, women reported side effects more often than men. The label also notes that women may respond to lower doses than men.³

Can niacin raise blood sugar?

At high doses, yes. NIH says nicotinic acid at 1.5 g a day or more is most likely to raise blood glucose, in people with or without diabetes.¹

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Sources

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.