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.
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 example, food industry wastewater—which may contain potato juice or milk—can be hazardous to the ecosystems of streams and rivers by consuming oxygen and causing eutrophication, but is nonhazardous to humans and not classified as a poison.
Uses of poison 
Throughout human history, intentional application of poison has been used as a method of murder, pest-control, suicide, and execution. As a method of execution, poison has been ingested, as the ancient Athenians did (see Socrates), inhaled, as with carbon monoxide or hydrogen cyanide (see gas chamber), or injected (see lethal injection). Many languages describe lethal injection with their corresponding words for "poison shot". Poison's lethal effect can be combined with its allegedly magical powers; an example is the Chinese gu poison. Poison was also employed in gunpowder warfare. For example, the 14th century Chinese text of the Huolongjing written by Jiao Yu outlined the use of a poisonous gunpowder mixture to fill cast iron grenade bombs.
The term poison with regard to biology and chemistry is often misused due to lack of a universal definition. Biologically speaking, any substance, if given in large enough amounts, is poisonous and can cause death. For instance, while botulinum toxin is lethal on the level of nanograms, a person would have to ingest kilograms worth of water to receive a lethal dose. While there may be a large disparity in this example, there are many substances used as medications where the LD50 is only one order of magnitude greater than the ED50 such as fentanyl. A better definition would distinguish between lethal substances that provide a therapeutic value and those that do not. The lack of a mathematical definition for the term poison impedes a universal definition.
Biological poisoning 
Acute poisoning is exposure to a poison on one occasion or during a short period of time. Symptoms develop in close relation to the exposure. Absorption of a poison is necessary for systemic poisoning. In contrast, substances that destroy tissue but do not absorb, such as lye, are classified as corrosives rather than poisons. Furthermore, many common household medications are not labeled with skull and crossbones, although they can cause severe illness or even death. In the medical sense, poisoning can be caused by less dangerous substances than those receiving the legal classification of "poison".
Chronic poisoning is long-term repeated or continuous exposure to a poison where symptoms do not occur immediately or after each exposure. The patient gradually becomes ill, or becomes ill after a long latent period. Chronic poisoning most commonly occurs following exposure to poisons that bioaccumulate, or are biomagnified, such as mercury and lead.
Contact or absorption of poisons can cause rapid death or impairment. Agents that act on the nervous system can paralyze in seconds or less, and include both biologically derived neurotoxins and so-called nerve gases, which may be synthesized for warfare or industry.
Inhaled or ingested cyanide, used as a method of execution in gas chambers, almost instantly starves the body of energy by inhibiting the enzymes in mitochondria that make ATP. Intravenous injection of an unnaturally high concentration of potassium chloride, such as in the execution of prisoners in parts of the United States, quickly stops the heart by eliminating the cell potential necessary for muscle contraction.
Most biocides, including pesticides, are created to act as poisons to target organisms, although acute or less observable chronic poisoning can also occur in non-target organism, including the humans who apply the biocides and other beneficial organisms. For example, the herbicide 2,4-D imitates the action of a plant hormone, to the effect that the lethal toxicity is specific to plants. Indeed, 2,4-D is not a poison, but classified as "harmful" (EU).
Many substances regarded as poisons are toxic only indirectly, by toxication. An example is "wood alcohol" or methanol, which is not poisonous itself, but is chemically converted to toxic formaldehyde and formic acid in the liver. Many drug molecules are made toxic in the liver, and the genetic variability of certain liver enzymes makes the toxicity of many compounds differ between individuals.
The study of the symptoms, mechanisms, treatment and diagnosis of biological poisoning is known as toxicology.
Exposure to radioactive substances can produce radiation poisoning, an unrelated phenomenon.
Initial management 
- Initial management for all poisonings includes ensuring adequate cardiopulmonary function and providing treatment for any symptoms such as seizures, shock, and pain.
- Poisons that have been injected (e.g. from the sting of poisonous animals) can be treated by binding the affected body part with a pressure bandage and by placing the affected body part in hot water (with a temperature of 50°C). The pressure bandage makes sure the poison is not pumped throughout the body and the hot water breaks down the poison. This treatment however only works with poisons that are composed of protein-molecules.
- Treatment of a recently ingested poison may involve gastric decontamination to decrease absorption. Gastric decontamination can involve activated charcoal, gastric lavage, whole bowel irrigation, or nasogastric aspiration. Routine use of emetics (syrup of Ipecac), cathartics or laxatives are no longer recommended.
- Activated charcoal is the treatment of choice to prevent poison absorption. It is usually administered when the patient is in the emergency room or by a trained emergency healthcare provider such as a Paramedic or EMT. However, charcoal is ineffective against metals such as sodium, potassium, and lithium, and alcohols and glycols; it is also not recommended for ingestion of corrosive chemicals such as acids and alkalis.
- Whole bowel irrigation cleanses the bowel, this is achieved by giving the patient large amounts of a polyethylene glycol solution. The osmotically balanced polyethylene glycol solution is not absorbed into the body, having the effect of flushing out the entire gastrointestinal tract. Its major uses are following ingestion of sustained release drugs, toxins that are not absorbed by activated charcoal (i.e. lithium, iron), and for the removal of ingested packets of drugs (body packing/smuggling).
- Gastric lavage, commonly known as a stomach pump, is the insertion of a tube into the stomach, followed by administration of water or saline down the tube. The liquid is then removed along with the contents of the stomach. Lavage has been used for many years as a common treatment for poisoned patients. However, a recent review of the procedure in poisonings suggests no benefit. It is still sometimes used if it can be performed within 1 hour of ingestion and the exposure is potentially life threatening.
- Nasogastric aspiration involves the placement of a tube via the nose down into the stomach, the stomach contents are then removed by suction. This procedure is mainly used for liquid ingestions where activated charcoal is ineffective, e.g. ethylene glycol poisoning.
- Emesis (i.e. induced by ipecac) is no longer recommended in poisoning situations, because vomiting is ineffective at removing poisons.
- Cathartics were postulated to decrease absorption by increasing the expulsion of the poison from the gastrointestinal tract. There are two types of cathartics used in poisoned patients; saline cathartics (sodium sulfate, magnesium citrate, magnesium sulfate) and saccharide cathartics (sorbitol). They do not appear to improve patient outcome and are no longer recommended.
Some poisons have specific antidotes:
Enhanced excretion 
- In some situations elimination of the poison can be enhanced using diuresis, hemodialysis, hemoperfusion, hyperbaric medicine, peritoneal dialysis, exchange transfusion or chelation. However, this may actually worsen the poisoning in some cases, so it should always be verified based on what substances are involved.
Further treatment 
- In the majority of poisonings the mainstay of management is providing supportive care for the patient, i.e. treating the symptoms rather than the poison.
See also 
- Agency for Toxic Substances and Disease Registry (ATSDR)
- Food taster
- Lethal injection
- List of extremely hazardous substances
- List of fictional toxins
- List of poisonings
- List of poisonous plants
- List of types of poison
- Poison ring
- Toxics use reduction
- Mr. Yuk
- "poison" at Dorland's Medical Dictionary
- Kautilya suggests employing means such as seduction, secret use of weapons, poison etc. S.D. Chamola, Kautilya Arthshastra and the Science of Management: Relevance for the Contemporary Society, p. 40. ISBN 81-7871-126-5.
- Kautilya urged detailed precautions against assassination—tasters for food, elaborate ways to detect poison. "Moderate Machiavelli? Contrasting The Prince with the Arthashastra of Kautilya". Critical Horizons, vol. 3, no. 2 (September 2002). Brill Academic Publishers. ISSN 1440-9917 (Print) 1568-5160 (Online). doi:10.1163/156851602760586671.
- Needham, Joseph (1986). Science and Civilization in China: Volume 5, Part 7. Taipei: Caves Books, Ltd. Page 180.
- Complete diving manual by Jack Jackson
- Chyka PA, Seger D, Krenzelok EP, Vale JA (2005). "Position paper: Single-dose activated charcoal". Clin Toxicol (Phila) 43 (2): 61–87. PMID 15822758.
- "Position paper: whole bowel irrigation". J Toxicol Clin Toxicol 42 (6): 843–854. 2004. doi:10.1081/CLT-200035932. PMID 15533024.
- Vale JA, Kulig K; American Academy of Clinical Toxicology; European Association of Poisons Centres and Clinical Toxicologist. (2004). "Position paper: gastric lavage". J Toxicol Clin Toxicol 42 (7): 933–943. doi:10.1081/CLT-200045006. PMID 15641639.
- American Academy of Clinical Toxicology; European Association of Poisons Centres Clinical Toxicologists (2004). "Position paper: Ipecac syrup". J Toxicol Clin Toxicol 42 (2): 133–143. doi:10.1081/CLT-120037421. PMID 15214617.
- Toxicology, American Academy of Clinical (2004). "Position paper: cathartics". J Toxicol Clin Toxicol 42 (3): 243–253. doi:10.1081/CLT-120039801. PMID 15362590.
- "WHO Disease and injury country estimates". World Health Organization. 2004. Retrieved Nov. 11, 2009.
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- Agency for Toxic Substances and Disease Registry
- American Association of Poison Control Centers
- American College of Medical Toxicology
- Clinical Toxicology Teaching Wiki
- Find Your Local Poison Control Centre Here (Worldwide)
- Poison Prevention and Education Website
- Cochrane Injuries Group, Systematic reviews on the prevention, treatment and rehabilitation of traumatic injury (including poisoning)
- Pick Your Poison—12 Toxic Tales by Cathy Newman