n. (Chem.) An organic acid (C6H5NO2), a carboxylated derivative of pyridine, obtained by the oxidation of nicotine; called also niacin and antipellagra vitamin, and vitamin B5. Chemically it is 3-pyridinecarboxylic acid. It is a cofactor in several enzymatic reactions, and is used to prevent a pellagra-like disease in dogs. [ MI11 ] Syn. -- 3-pyridinecarboxylic acid, niacin, antipellagra vitamin. [ 1913 Webster +PJC ]