A side effect of niacin can be skin flushing.
What is it?
Vitamin B3 is a water-soluble vitamin and unused vitamins leave your body through your urine. As your body does not store this vitamin you need a regular supply in your diet or through supplementation.
Vitamin B3 is made up of niacin (nicotinic acid) and niacinamide. A side effect of niacin is skin flushing, which can lead to non-adherence to treatment. Niacinamide does not have the same effect.
What it does
Niacin is crucial for conversion of food into energy, and helps maintain normal functioning of the skin, nerves and digestive system. It also helps regulate the body’s energy, blood sugar, antioxidant mechanisms, and detoxification reactions. It is essential to the development of a strong immune system. B3 supplementation has a favourable effect on several health conditions, including mental health.
Niacin, but not nicotinamide, is known to reduce serum cholesterol and is commonly prescribed with other lipid-lowering medications.  Research confirms that vitamin B3 is important to the successful treatment of multiple sclerosis and other nerve diseases. Research has reported that niacinamide can profoundly prevents the degeneration of demyelinated axons and improves the behavioural deficits. Niacin and its derivative nicotinamide are dietary precursors of nicotinamide adenine dinucleotide (NAD) and NAD influences mechanisms that maintain genomic stability. Loss of genomic stability is linked to DNA and chromosomal instability, which in turn are linked to cancer. 
Because neurologic disorders associated with pellagra resemble acute schizophrenia, niacin-based therapy was indicated as a treatment for schizophrenia. Abram Hoffer, MD, published extensively on the benefit of niacin on schizophrenia and other mental health conditions.  
Some common food sources of niacin include yeast, wheat bran, peanuts, wild and brown rice, sesame and sunflower seeds, pine nuts, buckwheat, whole-grains, red chili peppers, barley, almonds and split peas.
In many countries women and children, especially, suffer from severe deficiency symptoms caused by an inadequate supply of niacin, as well as a lack of iron, folate, and other micro-nutrients. While deficiency can be prevented by fortifying staple foods such as flour, orthomolecular scientists generally consider this to be a less than ideal source due to the negative effects of white flour on the body.
Recommended Dietary Allowance (RDA)
Canada’s RDA can be found here.
Molecular Dosage Range
Orthomolecular practitioners often use much higher levels than the RDA, sometimes up to several thousand mg/day, depending on circumstances.