A substance made or used when the body breaks down food, drugs or chemicals, or its own tissue (for example, fat or muscle tissue). This process, called metabolism, makes energy and the materials needed for growth, reproduction, and maintaining health. It also helps get rid of toxic substances. A metabolite is an intermediate or end product of metabolism. The term is usually used for small molecules.

Metabolites have various functions, including fuel, structure, signaling, stimulatory and inhibitory effects on enzymes, catalytic activity of their own (usually as a cofactor to an enzyme), defense, and interactions with other organisms (e.g. pigments, odorants, and pheromones). Primary metabolites are synthesized by the cell because they are indispensable for their growth. Significant representatives are amino acids, alcohols, vitamins (B2 and B12), polyols, organic acids, as well as nucleotides (e.g. inosine-5'-monophosphate and guanosine-5'-monophosphate).