Toxin

Toxins are substances that cause either permanent or reversible injury to the health of a living thing on contact or absorption, typically by interacting with biological macromolecules such as enzymes and receptors. The term is usually reserved for naturally produced substances that kill rapidly in small quantities, such as the bacterial proteins that cause tetanus and botulism. The word "toxic" is used more loosely and often applied to non-biological materials, as in "toxic waste" and "toxicology."

Injestable toxins are also often referred to as poisons, especially when intentionally administered by a human. Animal toxins 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 to defend itself while still alive. A single organism can be both.)

Many plants, animals and microorganisms generate toxins to discourage or kill predators. Food poisoning is a term for a broad range of illnesses that can result from eating food that is spoiled or tainted by bacterial toxins, such as botulinum and the so-called Shiga-like toxin secreted by the emergent E. coli strain E. coli O157:H7.

Naturally occurring or human-modified toxins may be intentionally released by humans in chemical warfare.

To see a list of biologically injurious substances, including toxins and other materials, as well as their effects, see poison.