Generally, niacin is not very toxic as this vitamin is excreted through the urine.
Taking more than 1 000 mg per day will merely cause the face to look flushed for a while.
But a niacin intake of 3 g per day can actually damage the liver – especially if the niacin is taken in the form of slow-releasing supplements.
An overdose of the vitamin may be especially dangerous in people suffering from asthma or peptic ulcers.
Taking large doses of niacin (to lower blood cholesterol levels, for example) should only be done under the guidance of a doctor.