Nicotinamide adenine dinucleotide (NAD) is a coenzyme central to metabolism. Found in all living cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an adenine nucleobase and the other, nicotinamide. NAD exists in two forms: an oxidized and reduced form, abbreviated as NAD⁺ and NADH (H for hydrogen), respectively.

In cellular metabolism, NAD is involved in redox reactions, carrying electrons from one reaction to another, so it is found in two forms: NAD⁺ is an oxidizing agent, accepting electrons from other molecules and becoming reduced; with H⁺, this reaction forms NADH, which can be used as a reducing agent to donate electrons. These electron transfer reactions are the main function of NAD. It is also used in other cellular processes, most notably as a substrate of enzymes in adding or removing chemical groups to or from proteins, in posttranslational modifications. Because of the importance of these functions, the enzymes involved in NAD metabolism are targets for drug discovery.

In organisms, NAD can be synthesized from simple building-blocks (de novo) from either tryptophan or aspartic acid, each a case of an amino acid. Alternatively, more complex components of the coenzymes are taken up from nutritive compounds such as nicotinic acid; similar compounds are produced by reactions that break down the structure of NAD, providing a salvage pathway that recycles them back into their respective active form.

In the name NAD⁺, the superscripted plus sign indicates the positive formal charge on one of its nitrogen atoms. A biological coenzyme that acts as an electron carrier in enzymatic reactions.

Some NAD is converted into the coenzyme nicotinamide adenine dinucleotide phosphate (NADP), whose chemistry largely parallels that of NAD, though its predominant role is as a coenzyme in anabolic metabolism. NADP is a reducing agent in anabolic reactions like the Calvin cycle and lipid and nucleic acid syntheses. NADP exists in two forms: NADP+, the oxidized form, and NADPH, the reduced form. NADP is similar to nicotinamide adenine dinucleotide (NAD), but NADP has a phosphate group at the C-2′ position of the adenosyl.

Nicotinamide adenine dinucleotide consists of two nucleosides joined by pyrophosphate. The nucleosides each contain a ribose ring, one with adenine attached to the first carbon atom (the 1' position) (adenosine diphosphate ribose) and the other with nicotinamide at this position.

The compound accepts or donates the equivalent of H⁻. Such reactions (summarized in formula below) involve the removal of two hydrogen atoms from a reactant (R), in the form of a hydride ion (H⁻), and a proton (H⁺). The proton is released into solution, while the reductant RH₂ is oxidized and NAD⁺ reduced to NADH by transfer of the hydride to the nicotinamide ring.

From the electron pair of the hydride ion, one electron is attracted to the slightly more electronegative atom of the nicotinamide ring of NAD⁺, becoming part of the nicotinamide moiety. The remaining hydrogen atom is transferred to the carbon atom opposite the N atom. The midpoint potential of the NAD⁺/NADH redox pair is −0.32 volts, which makes NADH a moderately strong reducing agent. The reaction is easily reversible, when NADH reduces another molecule and is re-oxidized to NAD⁺. This means the coenzyme can continuously cycle between the NAD⁺ and NADH forms without being consumed.