A metabolite is a substance that is involved in metabolism, which refers to the chemical processes that occur within cells. Metabolites can be either an intermediate or end product of metabolism, and the term is usually used for small molecules. They have various functions, including fuel, structure, signaling, catalytic activity, defense, and interactions with other organisms. Metabolites from chemical compounds, whether inherent or pharmaceutical, form as part of the natural biochemical process of degrading and eliminating the compounds. The rate of degradation of a compound is an important determinant of the duration and intensity of its action. Understanding how pharmaceutical compounds are metabolized and the potential side effects of their metabolites is an important part of drug discovery.

Metabolites can be classified into two major types: primary and secondary metabolites. Primary metabolites are those that are directly involved in the growth, development, and reproduction of an organism, and they are critical to the survival and fecundity of an organism. Examples of primary metabolites include ethanol, glutamic acid, aspartic acid, 5′ guanylic acid, acetic acid, lactic acid, and glycerol. Secondary metabolites are those that are not directly involved in the growth, development, and reproduction of an organism, but they may have other important functions. Examples of secondary metabolites include pigments, resins, terpenes, ergot, alkaloids, antibiotics, naphthalenes, nucleosides, quinolones, peptides, and growth hormones.