<unk>  is the deficiency of Niacin (Vitamin B3).
Niacin is an important factor for the production of NADH and NADPH, which are important for redox reactions.
Deficiency therefore affects tissues with high turnover, including skin, GI tract and brain
Niacin is either consumed in the diet or converted from tryptophan by the hepatic kynurenine pathway in the liver.
Neurological symptoms can also be exacerbated by supplementation by other B vitiamins, particularly B1, B2, B6, and B12.  Mechanism is unknown but may be secondary to the increased demand of NAD.
Deficiency therefore affects tissues with high turnover, including skin, GI tract and brain
Azathioprine
Chloramphenicol
Ethionamide
5-Fluorouracil
Isoniazid
6mercaptopurine
Pyrazinamide
Colitis
Celiac disease
Crohn's disease
Ulcerative colitis
Cirrhosis
Gastrectomy patients
Pellagra
Acute generalized exanthematous pustulosis
Strawberry tongue
Allergic reaction
Paronychia
Aphthous stomatitis
Body stuffing
Atopic dermatitis