Niacin flushes your system by increasing blood flow. It causes small veins, called capillaries to dilate, allowing blood to flow more freely.
This is considered a natural, helpful and beneficial effect of niacin, because toxins tend to build up in the stored fatty tissue of the body where larger vessels don't get to. The increase of blood flow helps flushing toxins out of the fatty tissue and out of your body. Such activation of the microcirculation causes the skin to warm up and look blushed, and may be accompanied by a kind of “tingling” sensation. It is only a temporary manifestation, usually lasting about ten minutes or so. This phenomenon, called niacin-flush, is totally harmless and indicative of an enhancing microcirculation.