The Collaborative International Dictionary of English v.0.48:

niacin \niacin\ n.
   A B vitamin (vitamin B5) essential for the normal function of
   the nervous system and the gastrointestinal tract. Called
   also nicotinic acid and antipellagra vitamin. Chemical
   formula C6H5NO2, chemically it is 3-pyridinecarboxylic
   acid.

   Syn: nicotinic acid.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

nicotinic acid \nic`o*tin"ic ac"id\, 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.

   Note: 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]

WordNet (r) 3.0 (2006):

niacin
    n 1: a B vitamin essential for the normal function of the
         nervous system and the gastrointestinal tract [syn:
         niacin, nicotinic acid]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "niacin":
   adermin, aneurin, antiberi-beri factor, ascorbic acid, axerophthol,
   biotin, carotene, cholecalciferol, choline, cobalamin,
   cryptoxanthin, cyanocobalamin, ergocalciferol, folic acid,
   hepatoflavin, inositol, lactoflavin, menadione, naphthoquinone,
   nicotinic acid, ovoflavin, pyridoxine, tocopherol, vitamin,
   vitamin B, vitamin Bc, vitamin D, vitamin G, vitamin H, vitamin K,
   vitamin M