A<unk>  is a chemical which is found in an organism but which is not normally produced or expected to be present in it.  It can also cover substances which are present in much higher concentrations than are usual. Specifically, drugs such as antibiotics are<unk> s in humans because the human body does not produce them itself nor would they be expected to be present as part of a normal diet. However, the term is also used in the context of pollutants such as dioxins and polychlorinated biphenyls and their effect on the biota. Natural compounds can also become<unk> s if they are taken up by another organism (e.g., uptake of natural human hormones by fish found downstream of sewage treatment plant outfalls).
Xenobiotic
Cathartic
Geothermal
Inhalant
Carcinogen
Viral vector
Phytochemical
Vector