None. Nicotinic acid and Niacin are synonyms, both terms indicating the same molecule. Nicotinic acid = Niacin = Vitamin B3 = Vitamin PP (Pellagra Preventing). Deficiency of Vitamin B3 is the cause of Pellagra, a dreadful desease, characterized by the famous 4 “D”s: dermatitis, diarrhea, dementia and death.
