Nicotinamide adenine dinucleotide, abbreviated NADh+, is a coenzyme found in all living cells. The compound is a dinucleotide, since it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an adenine base and the other nicotinamide. In metabolism, NAD+ is involved in redox reactions, carrying electrons from one reaction to another.  The coenzyme is, therefore, found in two forms in cells: NAD+ is an oxidizing agent, it accepts electrons from other molecules and becomes reduced. This reaction forms NADH,  (Nicotinamide adenine dinucleotide) which can then be used as a reducing agent to donate electrons. These electron transfer reactions are the main function of NAD+. However, it is also used in other cellular processes, the most notable one being a substrate of enzymes that add or remove chemical groups from proteins, in posttranslational modifications. Because of the importance of these functions, the enzymes involved in NAD+ metabolism are targets for drug discovery.

Moderate short-term (no more than three months) use generally does not cause side effects. But more than 10 mg can cause sleep difficulties, anxiety, nausea and fatigue. This may have to do with generating excess energy. People who take 10 milligrams a day can suffer from insomnia, mood swings and anxiety. But these side effects can disappear by themselves after they stop using them. Therefore, long-term use of nicotinamide adenine dinucleotide reductase is not recommended.