Vitamin B3

Niacin is a water-soluble vitamin used to form the coenzymes: nicotine adenine dinucleotide (NADH) and its ionized form (NAD+); and nicotine adenine dinucleotide phosphate (NADPH) and its ionized form (NADP+). These cofactors are used in many important redox and non-redox reactions throughout the body including energy production. 

Severe niacin deficiency causes the condition pellagra. 

Low levels are associated with fatigue, depression, memory loss, headache, hallucinations and diarrhoea.