A niacin flush is a common side effect of taking high doses of supplemental niacin, which is also known as vitamin B3 and is primarily used to treat high cholesterol levels. The flush occurs when capillaries expand, increasing blood flow to the skin’s surface, and typically sets in about 15–30 minutes after taking the supplement and tapers off after about an hour. The symptoms mainly affect the face and upper body and include reddening of the skin, tingling, burning, or itching. Although uncomfortable, the niacin flush is harmless and not dangerous to health.

Niacin flush can be prevented or minimized by taking aspirin or antihistamines 30 minutes before taking niacin, starting with a lower dose and gradually increasing it, taking niacin with food, or using extended-release versions of niacin supplements. It is important to note that niacin flush is not caused by the small amount of niacin found in most multivitamins, and that high doses of niacin should only be taken under the supervision of a doctor.