Metabolites are broadly divided into two classes: primary metabolites, which are directly involved in normal growth, development, and reproduction (such as amino acids, nucleotides, and organic acids); and secondary metabolites, which are not essential to those core functions but serve other roles , defence, signalling, or ecological interaction , and include compounds such as alkaloids, terpenes, and flavonoids. Many plant-derived medicines of interest in phytochemistry fall into the secondary-metabolite category.
The term carries a further distinction in pharmacology and toxicology. When an exogenous compound (a drug, plant extract, or synthetic substance) enters the body, the liver and other tissues chemically transform it via Phase I and Phase II reactions. The resulting molecules are called drug metabolites or xenobiotic metabolites. These may be pharmacologically active, inactive, or in some cases more biologically potent than the parent compound , a fact that complicates straightforward interpretation of preclinical data.
In cannabinoid science specifically, the distinction matters considerably. Δ9-tetrahydrocannabinol (THC), for instance, is metabolised in the liver primarily to 11-hydroxy-THC and then to 11-nor-9-carboxy-THC (THC-COOH), a pharmacologically inactive but analytically persistent carboxylic acid. The presence of a metabolite rather than a parent compound is what most standard urine immunoassay screens detect, which has direct implications for regulatory and workplace-testing frameworks in Australia.
The systematic study of all metabolites present in a biological sample at a given time is called metabolomics, a field that has expanded considerably since the Human Metabolome Database (HMDB) began cataloguing identified human metabolites in the early 2000s. As of the HMDB's 2023 update, the database lists more than 220,000 metabolite entries, though the confirmed and quantified subset remains substantially smaller.
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