noun \ˈnī-ə-sən\

Definition of NIACIN

: a crystalline acid C6H5NO2 that is a member of the vitamin B complex occurring usually in the form of a complex of niacinamide in various animal and plant parts (as blood, liver, yeast, bran, and legumes) and is effective in preventing and treating human pellagra and blacktongue of dogs—called also nicotinic acid

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