What causes the flush from niacin?
Vitamin supplements containing niacin (vitamin B3) causes a release of histamine from mast cells the immune cells in tissues that respond to allergies. This causes a flush of the skin almost like an allergic response, usually lasting for 20 to 40 minutes. If you take niacin regularly (twice a day or more) the flush usually stops because you have depleted your mast cells of histamine. The flush is not harmful, but some people find it uncomfortable (especially in dress clothes). Others find that they enjoy the flush. Timed-release niacin usually causes less of a flush, but it may still happen.
