Xenobiotic

Xenobiotic is a chemical compound that is foreign to a biological system. The term is used in pharmacology, toxicology, and environmental science to describe substances not endogenously produced by the organism, nor normally present in its metabolism. Examples include pharmaceutical drugs, pesticides, industrial chemicals, food additives, and environmental pollutants.

Most xenobiotics are subject to metabolism and excretion to reduce toxicity and promote elimination. In many organisms, liver enzymes—especially the cytochrome P450 family—initiate oxidative phase I reactions that introduce or reveal functional groups. Phase II reactions then conjugate the compound (for example glucuronidation, sulfation, or glutathione conjugation) to increase water solubility. The resulting products are excreted by the kidneys or into bile. Some xenobiotics can be activated to more reactive or toxic forms, a process known as bioactivation.

In pharmacology, xenobiotics include therapeutic drugs, and understanding their absorption, distribution, metabolism, and excretion (ADME) is essential for dosing and safety. Xenobiotic interactions, such as inhibition or induction of metabolic enzymes, can alter the fate of other xenobiotics and endogenous substrates. In environmental science, xenobiotics can be persistent and may bioaccumulate or biomagnify in food chains, prompting regulatory assessment of exposure, toxicity, and environmental fate.

Etymology: from Greek xenos “foreign” and bios “life.” The concept helps compare endogenous substances with externally derived compounds and informs risk assessment, pharmacokinetics, and toxicology.