Niacin makes your blood vessels widen and thus improves circulation.  Together with riboflavin ( vitamin B2) and pyridoxine ( vitamin B6) it promotes healthy skin, and keep the nervous and digestive systems running smoothly.

Alcohol inhibits niacin absorption, and diets that lack protein are probably niacin deficient.  Niacin deficiency may result in canker sores, dementia, depression, diarrhea, dizziness, fatigue, halitosis, headaches, and indigestion, insomnia, limb pains, loss of appetite, low blood sugar, muscular weakness, skin eruptions, and inflammation.

Potentially fatal Pellagra is caused by extreme lack of niacin, and is characterized by diarrhoea, mental disorders, depression, and skin problems.

The  RDA for niacin is:
18 milligrams for adult men
14 milligrams for adult women
18 milligrams for nursing mothers and 20 milligrams if pregnant.