Metabolites are the products and intermediates of cellular metabolism, catalyzed by various enzymes that occur naturally within cells. The term metabolites are usually used for small molecules. The various functions of metabolites include; fuel, structure, signalling, catalytic activity, defence and interactions with other organisms. Metabolites are not only substrates in metabolic reactions, but also signaling molecules controlling a wide range of cellular processes. Metabolites can have a multitude of functions, including energy conversion, signaling, epigenetic influence, and cofactor activity.

The process of metabolizing drugs in the body occurs in the same manner. For instance , THC in Marijuana is metabolized into tetrahydrocannabinol carboxylic acid (THC-COOH), the primary metabolite of Marijuana. Cocaine is metabolized into benzoylecgonine as the major metabolite. The presence of a drug metabolite is a reliable indicator of the parent drug intake in the body.