Xenobiotics are any chemical compounds that are found in a living organism, but which are foreign to that organism, in the sense that it does not normally produce the compound or consume it as part of its diet. For example, in humans, most drugs are part of this category, since people don't produce them naturally, or consume them under normal circumstances. Xenobiotics can also be defined as substances that are present in higher-than-normal concentrations, or ones that are entirely artificial and did not exist before they were produced synthetically by humans.
A compound that is normal to one organism may be a xenobiotic to another. A commonly used example of this is the effect experienced by fish that live downstream from the outlet of a sewage treatment plant. Hormones produced by humans may be present, even in treated water, and these compounds are foreign as far as fish are concerned.
When an animal produces a toxin as a defense mechanism against predators, these toxins can also be thought of as xenobiotics from the point of view of the predator. Predators can also evolve defenses against these toxins, however. One interesting case in which this has occurred is in the rough-skinned newt and its predator, the garter snake. Tetrodotoxin is a xenobiotic made in the body of the newt, which is usually very poisonous to other animals, but the garter snake has evolved with a resistance to it in order to be able to continue to prey on the newt. It is speculated that as the newt species produced higher and higher concentrations of the poison over generations, the garter snake developed a progressively stronger resistance to it.
When human bodies encounter a such a substance, they remove it through a process called xenobiotic metabolism. Through these processes, the chemical structure of the compound is modified and broken down to either be put to use by the body, or to be detoxified, in the case of some poisons. Sometimes, it is these chemical pathways and their reactions that can be fatal, as is the case with poisoning deaths and harmful drug interactions.
The metabolic pathways that the body uses to process foreign substances have long been a subject of great interest on the part of medical science. It is largely through studying these pathways that new drugs are proposed and developed, especially in the case of chemotherapy drugs used in cancer treatment. They are also important in studying the potential effects of pollutants on an environment, to see whether the chemical will be broken down or remain in the environment and cause harm.