Vitamin B₃, Niacin, is an essential micronutrient and belongs to vitamins of B-complex group which are water-soluble.
Other forms of niacin are nicotinic acid and nicotinamide. These forms may not be confused with nicotine, present in tobacco, which is totally a different chemical.
Niacin, like other B vitamins, helps in converting fat, proteins and carbohydrates into energy.
Niacin as a coenzyme helps more than hundred enzymes in bringing about redox reactions (oxidation-reduction).
Niacin has an active role in the synthesis of cholesterol and fatty acids.
Niacin helps in the functions of the digestive systems like promoting appetite, digestion and assimilation.
A number of helpful roles have been attributed to niacin like cell differentiation, DNA repair, DNA transcription and energy release in the cells.
Niacin is believed to help in preventing the formation and growth of cancer cells.
Niacin helps in removing toxins and harmful chemicals from the human body.
Tryptophan an amino acid found in human beings converts to niacin under certain circumstances.
Digestive system, circulatory system and nervous system are kept in healthy condition with the help of niacin.
Niacin is a memory enhancer and helps in the treatment of mental illness along with other specific medicines.
Niacin is found to lower bad cholesterol and increase good cholesterol.
Vitamin B₃ is believed to help in preventing and treating heart diseases like Atherosclerosis.

Deficiency
Corn is very low in niacin and in places where corn is the staple food, the population is found to be deficient in niacin.
Mild deficiency of niacin slows down body's metabolism and the deficient persons particularly become intolerant to cold.
Pellagra is a disease caused by severe deficiency of vitamin B₃ in which dermatitis, diarrhoea and mental illness is caused.
Skin inflammations, sores, pains, weakness, low blood sugar, headaches, dementia, depression, diarrhoea and loss of appetite are some of the symptoms of deficiency of vitamin B₃

Toxicity
Though higher doses are found to treat many ailments, prolonged use of very high doses are found to cause liver damage.

Sources
Meat, fish, liver, poultry, whole grains and cereals (except corn), vegetables and fruits are good sources of vitamin B₃