Why You Need It
Niacin is the building block for NAD and NADP coenzymes, which take part in hundreds of metabolic reactions. Your body can also make some niacin from the amino acid tryptophan, which is why protein-rich foods help cover your needs.
Too Little or Too Much
Severe deficiency (called pellagra) damages the skin, gut, and nervous system — it's rare today. High doses of nicotinic acid in supplements can cause skin flushing and warmth, and long-term use may put stress on the liver, so don't take them without a doctor's advice.