For other uses see Toxin (disambiguation).
A toxin (Greek:, toxikon, lit. (poison) for use on arrows) is a poisonous substance produced by living cells or organisms. (Although technically man is a living organism, man-made substances created by artificial processes usually aren't considered toxins by this definition.)
For a toxic substance not produced by living organisms, "toxicant" is the more appropriate term, and "toxics" is an acceptable plural.
Toxins can be small molecules, peptides, or proteins and are capable of causing disease on contact with or absorption by body tissues by interacting with biological macromolecules such as enzymes or cellular receptors. Toxins vary greatly in their severity, ranging from usually minor and acute (as in a bee sting) to almost immediately deadly (as in botulinum toxin).
Toxins are often distinguished from other chemical agents by their method of production- the word toxin does not specify method of delivery (compare with venom and (the narrower meaning) of poison). It simply means it is a biologically produced poison.There was an ongoing dispute between NATO and the Warsaw Pact over whether to call a toxin a biological or chemical agent, in which the former opted for the latter, and vice versa.
According to a International Committee of the Red Cross review of the Biological Weapons Convention, "Toxins are poisonous products of organisms; unlike biological agents, they are inanimate and not capable of reproducing themselves." and "Since the signing of the Convention, there have been no disputes among the parties regarding the definition of biological agents or toxins..."
According to Title 18 of the United States Code, "...the term "toxin" means the toxic material or product of plants, animals, microorganisms (including, but not limited to, bacteria, viruses, fungi, rickettsiae or protozoa), or infectious substances, or a recombinant or synthesized molecule, whatever their origin and method of production..."
The term "biotoxin" is sometimes used to explicitly confirm the biological origin.
Biotoxins vary greatly in purpose and mechanism, and can be highly complex (the venom of the cone snail contains dozens of small proteins, each targeting a specific nerve channel or receptor), or relatively small protein.
Biotoxins in nature have two primary functions:
Some of the more well known types of biotoxins include:
In these contexts, it can sometimes explicitly include contaminants that are man-made,  which contradicts most formal definitions of the term "toxin". Because of this, when encountering the word "toxin" outside of microbiological contexts, it is important to confirm what the researcher means by the use of the term.
When used non-technically, the term "toxin" is often applied to any toxic substances. Toxic substances not of biological origin are more properly termed poisons. Many non-technical and lifestyle journalists also follow this usage to refer to toxic substances in general, though some specialist journalists at publishers such as the BBC and The Guardian maintain the distinction that toxins are only those produced by living organisms.
In the context of alternative medicine the term is often used non-specifically to refer to any substance claimed to cause ill health, ranging anywhere from trace amounts of pesticides to common food items like refined sugar or additives like artificial sweeteners and MSG.