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Nicotinamide, ${\mathrm{C}}_6{\mathrm{H}}_6{\mathrm{N}}_2\mathrm{O}$, is the amide of nicotinic acid, with a pyridine ring bearing a carboxamide group. It is one form of vitamin B3 and supplies the nicotinamide unit of nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate.^[ref]

These coenzymes connect its structure to metabolism: the nicotinamide ring participates in reversible electron transfer between oxidized and reduced forms. Incorporation into a coenzyme is different from the free nicotinamide molecule itself carrying out every associated reaction.^[ref]

Nicotinic acid, ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{N}{\mathrm{O}}_2$, is pyridine-3-carboxylic acid: a pyridine ring bearing a carboxyl group at position 3. It is one form included under vitamin B3, and can contribute to production of the coenzymes nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate.^[ref][ref]

The related nicotinamide has a carboxamide group in place of the acid group. Their related names reflect related structures, while the different functional groups make them distinct chemical compounds. “Niacin” can be used broadly for the vitamin family, so an experiment reporting a particular chemical form should specify which molecule was used.