NAD+ is a dinucleotide built from a nicotinamide riboside and an adenosine ribose joined through a pyrophosphate bridge. It is the oxidised member of the NAD+/NADH redox couple and serves as the electron acceptor for several hundred dehydrogenase reactions, including those of glycolysis, the tricarboxylic acid cycle and fatty acid oxidation. It is a coenzyme and a small molecule; it contains no amino acids and is not a peptide.

Separately from that redox role, NAD+ is consumed as a substrate rather than used catalytically by three enzyme families: the sirtuin deacylases, the poly(ADP-ribose) polymerases and CD38. Those reactions cleave the molecule at the nicotinamide-ribose bond, so cellular NAD+ must be continuously regenerated through the salvage pathway from nicotinamide. That consumption relationship is the reason the molecule appears in cellular-ageing and metabolic research at all, and it is also the link to NNMT inhibitors such as 5-Amino-1MQ, which act on the same nicotinamide pool.

NAD+ is not an approved medicine in any jurisdiction. It has been formally considered by the FDA in the compounding context: the Pharmacy Compounding Advisory Committee recommended in 2017 against adding it to the 503A Bulks List, citing instability and insufficient literature, and it currently sits on FDA's 503A Category 2 list of bulk drug substances that raise significant safety risks. Practically, the material is hygroscopic and degrades on exposure to light, moisture and warmth, so mass and purity on a certificate of analysis describe the substance at release and not necessarily on arrival.