An extreme deficiency of niacin causes the mortal disease called pellagra, with mental symptoms such as hallucinations and dementia, skin rashes, and diarrhea.
Lesser deficiencies give rise to depression, irritability, insomnia, backaches, headaches and so on.
The effect is particularly noticeable in the hands and forearms. Some may find this uncomfortable. The effect soon wears off. Administration of nicotinamide, another form of Niacin, has been reported to result in some depressions among mental patients treated for niacin deficiency. This Vitamin is water soluble.