In the context of biology, poisons are substances that cause disturbances to organisms,  usually by chemical reaction or other activity on the molecular scale, when a sufficient quantity is absorbed by an organism. The fields of medicine (particularly veterinary) and zoology often distinguish a poison from a toxin, and from a venom. Toxins are poisons produced by some biological function in nature, and venoms are usually defined as toxins that are injected by a bite or sting to cause their effect, while other poisons are generally defined as substances absorbed through epithelial linings such as the skin or gut. Some poisons are also toxins, usually referring to naturally produced substances, such as the bacterial proteins that cause tetanus and botulism. A distinction between the two terms is not always observed, even among scientists.
Animal poisons that are delivered subcutaneously (e.g. by sting or bite) are also called venom. In normal usage, a poisonous organism is one that is harmful to consume, but a venomous organism uses poison (venom) to kill its prey or defend itself while still alive. A single organism can be both venomous and poisonous.
The derivative forms "toxic" and "poisonous" are synonymous.
In nuclear physics, a poison is a substance that obstructs or inhibits a nuclear reaction. For an example, see nuclear poison.
The term "poison" is often used colloquially to describe any harmful substance, particularly corrosive substances, carcinogens, mutagens, teratogens and harmful pollutants, and to exaggerate the dangers of chemicals. Paracelsus, the father of toxicology, once wrote: "Everything is poison, there is poison in everything. Only the dose makes a thing not a poison." The legal definition of "poison" is stricter. A medical condition of poisoning can also be caused by substances that are not legally required to carry the label "poison". Environmentally hazardous substances are not necessarily poisons and vice versa. For...
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