A toxin (from Ancient Greek: τοξικόν toxikon) is a poisonous substance produced within living cells or organisms; synthetic substances created by artificial processes are thus excluded. The term was first used by organic chemist Ludwig Brieger (1849–1919).
Toxins can be small molecules, peptides, or proteins that are capable of causing disease on contact with or absorption by body tissues 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—all substances that can also cause disturbances to organisms). It simply means it is a biologically produced poison. There was an ongoing terminological dispute between NATO and the Warsaw Pact over whether to call a toxin a biological or chemical agent, in which the NATO opted for biological agent, and the Warsaw Pact, like most other countries in the world, for chemical agent.
According to an 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..."
A rather informal terminology of individual toxins relates them to the anatomical location where their effects are most notable:
- Hemotoxin, causes destruction of red blood cells (hemolysis)
- Phototoxin, causes dangerous photosensitivity
Related terms are:
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:
- Predation (spider, snake, scorpion, jellyfish, wasp)
- Defense (bee, ant, termite, honeybee, wasp, poison dart frog)
Some of the more well known types of biotoxins include:
- Cyanotoxins, produced by cyanobacteria
- Hemotoxins target and destroy red blood cells, and are transmitted through the bloodstream. Organisms that produce hemotoxins include:
- Necrotoxins cause necrosis (i.e., death) in the cells they encounter and destroy all types of tissue. Necrotoxins spread through the bloodstream. In humans, skin and muscle tissues are most sensitive to necrotoxins. Organisms that possess necrotoxins include:
- Neurotoxins primarily affect the nervous systems of animals. Organisms that possess neurotoxins include:
- Cytotoxins are toxic at the level of individual cells, either in a non-specific fashion or only in certain types of living cells:
- Ricin is a plant toxin found in the castor bean plant
- Apitoxin, the honey bee venom
- Mycotoxins are toxins produced by fungi. They are a common source of toxins in grains and other foods.
The term "environmental toxin" can sometimes explicitly include synthetic contaminants  such as industrial pollutants and other artificially made toxic substances. As this contradicts most formal definitions of the term "toxin", it is important to confirm what the researcher means when encountering the term outside of microbiological contexts.
Environmental toxins from food chains that may be dangerous to human health include:
- Paralytic shellfish poisoning (PSP)
- Amnesic shellfish poisoning (ASP)
- Diarrheal shellfish poisoning (DSP)
- Neurotoxic shellfish poisoning (NSP) 
Finding information about toxins
The Toxicology and Environmental Health Information Program (TEHIP) at the United States National Library of Medicine (NLM) maintains a comprehensive toxicology and environmental health web site that includes access to toxins-related resources produced by TEHIP and by other government agencies and organizations. This web site includes links to databases, bibliographies, tutorials, and other scientific and consumer-oriented resources. TEHIP also is responsible for the Toxicology Data Network (TOXNET), an integrated system of toxicology and environmental health databases that are available free of charge on the web.
TOXMAP is a Geographic Information System (GIS) that is part of TOXNET. TOXMAP uses maps of the United States to help users visually explore data from the United States Environmental Protection Agency's (EPA) Toxics Release Inventory and Superfund Basic Research Programs.
Computational resources for prediction of toxic peptides and proteins
One of the bottlenecks in peptide/protein-based therapy is their toxicity. Recently, in silico models for predicting toxicity of peptides and proteins, have been developed by Gajendra Pal Singh Raghava's group, predict toxicity with reasonably good accuracy. The prediction models are based on machine learning technique and quantitative matrix using various properties of peptides. The prediction tool is freely accessible to public in the form of web sever - ToxinPred at http://crdd.osdd.net/raghava/toxinpred/.
Misuse of term
When used non-technically, the term "toxin" is often applied to any toxic substance. Toxic substances not directly of biological origin are also termed poisons and many non-technical and lifestyle journalists follow this usage to refer to toxic substances in general.
In the context of alternative medicine the term is often used 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 such as monosodium glutamate (MSG).
- Insect toxins
- Toxin-antitoxin system
- List of fictional toxins
- List of highly toxic gases
- Microbial toxins
- Toxicophore, feature or group within a chemical structure that is thought to be responsible for the toxic property
- "toxin" at Dorland's Medical Dictionary
- "toxin - Definition from the Merriam-Webster Online Dictionary". Retrieved 13 December 2008.
- "The Biological Weapons Convention - An overview". Retrieved 13 December 2008.
- "U.S. Code". Retrieved 13 December 2008.
- "biotoxin - Definition from the Merriam-Webster Online Dictionary". Retrieved 13 December 2008.
- "biotoxin" at Dorland's Medical Dictionary
- Proft T (editor) (2009). Microbial Toxins: Current Research and Future Trends. Caister Academic Press. ISBN 978-1-904455-44-8.
- Grigg J (March 2004). "Environmental toxins; their impact on children's health". Arch. Dis. Child. 89 (3): 244–50. doi:10.1136/adc.2002.022202. PMC 1719840. PMID 14977703.
- Vale, Carmen; Alfonso, Amparo; Vieytes, Mercedes R.; Romarís, Xosé Manuel; Arévalo, Fabiola; Botana, Ana M.; Botana, Luis M. (2008). "In Vitro and in Vivo Evaluation of Paralytic Shellfish Poisoning Toxin Potency and the Influence of the pH of Extraction". Analytical chemistry (American Chemical Society) 80 (5): 1770–1776. doi:10.1021/ac7022266. PMID 18232710.
- Oikawa, Hiroshi; Fujita, Tsuneo; Saito, Ken; Satomi, Masataka; Yano, Yutaka (2008). "Difference in the level of paralytic shellfish poisoning toxin accumulation between the crabs Telmessus acutidens and Charybdis japonica collected in Onahama, Fukushima Prefecture". Fisheries Science (Springer) 73 (2): 395–403. doi:10.1111/j.1444-2906.2007.01347.x.
- Abouabdellah, Rachid; Taleb, Hamid; Bennouna, Asmae; Erler, Katrin; Chafik, Abdeghani; Moukrim, Abdelatif (2008). "Paralytic shellfish poisoning toxin profile of mussels Perna perna from southern Atlantic coasts of Morocco". Toxin (Elsevier) 51 (5): 780–786. doi:10.1016/j.toxicon.2007.12.004. PMID 18237757.
- Wang, Lin; Liang, Xu-Fang; Zhang, Wen-Bing; Mai, Kang-Sen; Huang, Yan; Shen, Dan (2009). "Amnesic shellfish poisoning toxin stimulates the transcription of CYP1A possibly through AHR and ARNT in the liver of red sea bream Pagrus major". Marine Pollution Bulletin (Elsevier) 58 (11): 1643–1648. doi:10.1016/j.marpolbul.2009.07.004. PMID 19665739.
- Wang, Lin; Vaquero, E.; Leão, J. M.; Gogo-Martínez, A.; Rodríguez Vázquez, J. A. (2001). "Optimization of conditions for the liquid chromatographic-electrospray lonization-mass spectrometric analysis of amnesic shellfish poisoning toxins". Chromatographia (Vieweg Verlag) 53 (1): S231–S235. doi:10.1007/BF02490333.
- Mouratidou, Theoni; Kaniou-Grigoriadou, I.; Samara, C.; Kouimtzis, T. (2006). "Detection of the marine toxin okadaic acid in mussels during a diarrhetic shellfish poisoning (DSP) episode in Thermaikos Gulf, Greece, using biological, chemical and immunological methods". Science of the Total Environment (Elsevier) 366 (2 – 3): 894–904. doi:10.1016/j.scitotenv.2005.03.002. PMID 16815531.
- Doucet, Erin; Ross, Neil N.; Quilliam, Michael A. (2007). "Enzymatic hydrolysis of esterified diarrhetic shellfish poisoning toxins and pectenotoxins". Analytical and Bioanalytical Chemistry (Springer) 389 (1): 335–342. doi:10.1007/s00216-007-1489-3. PMID 17661021.
- Poli, Mark A.; Musser, Steven M.; Dickey, Robert W.; Eilers, Paul P.; Hall, Sherwood (2000). "Neurotoxic shellfish poisoning and brevetoxin metabolites: a case study from Florida". Toxicon (Elsevier) 38 (7): 981–993. doi:10.1016/S0041-0101(99)00191-9. PMID 10728835.
- Morohashi, Akio; Satake, M.; Murata, K.; Naoki, H.; Kaspar, H.; Yasumoto, T. (1995). "Brevetoxin B3, a new brevetoxin analog isolated from the greenshell mussel perna canaliculus involved in neurotoxic shellfish poisoning in new zealand". Tetrahedron Letters (Elsevier) 36 (49): 8995–8998. doi:10.1016/0040-4039(95)01969-O.
- Morohashi, Akio; Satake, Masayuki; Naoki, Hideo; Kaspar, Heinrich F.; Oshima, Yasukatsu; Yasumoto, Takeshi (1999). "Brevetoxin B4 isolated from greenshell mussels Perna canaliculus, the major toxin involved in neurotoxic shellfish poisoning in New Zealand". Tetrahedron Letters (Natural Toxins) 7 (2): 45–48. doi:10.1002/(SICI)1522-7189(199903/04)7:2<45::AID-NT34>3.0.CO;2-H. PMID 10495465. Retrieved 15 February 2010.
- Prince of Wales criticised for dodgy detox product, Sense About Science, March 2009
- T3DB: Toxin-target database
- ATDB: Animal toxin database
- Society of Toxicology
- The Journal of Venomous Animals and Toxins including Tropical Diseases
- ToxSeek: Meta-search engine in toxicology and environmental health
- Website on Models & Ecotoxicology