There are two types of Niacin, Nicotinic Acid and Niacinamide. To achieve the Niacin "flush", which will happen by taking a megadose (about 300-900 mg. and all B vitamins are water-soluble so it is very difficult to take enough to cause any type of damage to your body). the "flush" will happen with Nicotinic Acid because you will dilate the capillaries, therefore enabling a mad rush of blood to flow in a way that it normally doesn't. This is what causes the hot flashes and light headedness (which I actually enjoy). And this doesn't really detox a person but rather allows toxins that are stored deep inside body tissue to be put into the circulatory system and allow the liver to filter the toxins that were trapped way down, out of the body. So it goes without saying to not Niacin flush the day of your UA. Niacinamide will NOT do this because it is a synthetic vitamin B3 and is manufactured to be time released in the body to specifically not give a "flush" of the skin. But most of this can be avoided simply by adding more cayenne pepper and ginger to your diet, both help very much with circulation.