Many deadly poisons are produced by plants, animals, fungi and microorganisms. These toxins often serve an evolutionary purpose, such as a defence against predators.
Some biotoxins have evolved to target specific physiological systems, making them highly effective – and in some cases, deadly – if encountered by humans in even trace amounts.
- 10 of the world’s most poisonous plants - including the world's deadliest tree, where every part, from the bark to the sap, is highly toxic
- Why are nature's deadliest poisons so lethal?We explain the difference between venoms and toxins - and why some are so severe they kill
Assessing and ranking toxicity is not easy because of differences between species, the varying state of a toxin and how it enters the body (oral, intravenous, intraperitoneal, inhalation).
Lethal dose 50 (LD50) is a measure of how much of a substance is lethal to half of a group of test animals. Here, it’s shown as milligrams (mg) of the substance per kilogram (kg) of body mass and any comparisons should be made with caution.
Deadliest poisons in nature
Tetrodotoxin
Found mainly in the liver and sex organs of some fish, including pufferfish, globefish, and toadfish, which belong to the order Tetraodontiformes, tetrodotoxin is a very powerful toxin that can be fatal. It also occurs in some amphibian, octopus (including blue-ringed octopus, one of the deadliest sea creatures), and shellfish species.
According to the U.S Centres for Disease Control and Prevention, “Human poisonings occur when the flesh and/or organs of the fish are improperly prepared and eaten. Tetrodotoxin interferes with the transmission of signals from nerves to muscles and causes an increasing paralysis of the muscles of the body.”
For this reason, the restaurant preparation of 'fugu' (a pufferfish delicacy) in Japan is strictly controlled by law and only specially trained chefs can prepare the fish. The LD50 of tetrodoxin in mice (oral) is 0.000334mg/kg.
Abrin
Abrin has a LD50 in mice (intravenous) of 0.00002mg/kg. It comes from the seeds of the rosary pea plant, which grows in tropical areas around the world. The seeds are red with a black spot on one end and just one contains enough abrin to kill an adult, although accidental exposure to abrin is unlikely.
According to the US Centers for Disease Control and Prevention (CDC), “Abrin makes people sick by getting into the cells of people's bodies and stopping cells from making proteins. Without the proteins, cells die. Eventually this hurts the whole body and can lead to death.”
Botulinum
Clostridium botulinum is a bacterium that produces lethal toxins (botulinum toxins) under low-oxygen conditions. The toxins block nerve functions and can lead to respiratory and muscular paralysis. According to the World Health Organization (WHO), “Botulinum toxins are one of the most lethal substances known”. The LD50 of botulinum in humans (inhalation) is estimated to be 0.000001 to 0.000003mg/kg.
Human botulism is mainly a foodborne intoxication when botulism toxins are ingested through improperly processed food, but it is relatively rare. It can also be caused by intestinal infection of C. botulinum in infants, wound infections, and by inhalation.
Another toxin that is a member of the Clostridium family is Clostridium tetani a soil soil bacteria that causes the toxin, tetanospasmin and the deadly disease tetanus, which you can be vaccinated against.
Maitotoxin
Maitotoxin is a highly potent marine poison produced by dinoflagellate (plankton) Gambierdiscus toxicus.
The species is found in tropical and subtropical areas of the Pacific Ocean and is known for its involvement in ciguatera fish poisoning, caused by eating tropical reef fish that contain the toxin. Maitotoxin affects cell function in the human body and its LD50 in mice (intraperitoneal) is 0.00005mg/kg.
Amatoxin
One of the deadliest mushrooms in the world, the death cap (Amanita phalloides) is responsible for approximately 90 per cent of mushroom-related fatalities worldwide, reports The Guardian. The mushroom produces three major classes of toxins: amatoxins, phallotoxins, and virotoxins, with the amatoxin α-Amanitin being the most lethal.
Amatoxins disrupt protein synthesis in cells, primarily targeting the liver. Ingestion of the death cap can lead to liver failure, kidney failure, encephalopathy, and death within one to seven days. The LD50 of α-Amanitin in humans (oral) is estimated to be 0.0001mg/kg. While an antidote exists, it must be administered before the toxins bind to cells to be effective.
Digoxin
Foxgloves (Digitalis species) contain cardiac glycosides, primarily digoxin and digitoxin, which can be deadly to humans if ingested. While foxglove derivatives are known to treat certain heart conditions, accidental or intentional ingestion of the plant can cause nausea, vomiting, diarrhoea, confusion, visual disturbances and heart arrhythmias. The LD50 of digitoxin in cats (intravenous) is 0.00018mg/kg.
Aflatoxin
Amongst the most poisonous mycotoxins are aflatoxins that are produced by certain fungi (Aspergillus flavus and Aspergillus parasiticus). These species affect crops, including cereal, oilseed, spice and tree nut.
The toxins can also be found in the milk of animals that are fed contaminated feed. The WHO states, “Large doses of aflatoxins can lead to acute poisoning… and can be life threatening, usually through damage to the liver. Aflatoxins have also been shown to… damage DNA.” Exposure is associated with an increased risk of cancer. LD50 of aflatoxin B1 (the most toxic form) in mice (intraperitoneal) is 0.009mg/kg.
Strychnine
The source of strychnine is the strychnine tree (Strychnos nux-vomica), which is native to India and Southeast Asia. “Only a little strychnine is needed to cause serious effects in people,” states the CDC. The toxin affects the nervous system, resulting in severe spasms. The LD50 of strychnine in humans (oral) is approximately 0.0015 to 0.002mg/kg.
Historically, strychnine was administered in small amounts to treat a broad range of ailments but is now used mainly as a pesticide. “Strychnine is also sometimes found mixed with drugs such as LSD, heroin and cocaine.”
Batrachotoxin
Batrachotoxin is a toxin secreted by poison dart frogs belonging to the Phyllobates genus and is also found in the feathers and skin of some birds of the genera Pitohui and Ifrita. Research suggests the frogs and birds get the toxins from dietary sources. Batrachotoxin is one of the most potent poisons in nature as it causes heart failure and affects the nervous system, resulting in paralysis and seizures. The LD50 of batrachotoxin in mice (intravenous) is 0.002mg/kg.
Ricin
Ricin comes from the beans of the castor plant (Ricinus communis), which is a species of perennial flowering plant in the spurge family. The plant is widely cultivated as its beans are used to make castor oil; during processing heat deactivates the ricin in the seeds, making the oil safe to use. The LD50 for ricin in mice (inhalation) is 0.003mg/kg.
According to Minnesota Department of Health, “It’s very unlikely that anyone would ever be poisoned with ricin by accident.” If ingested, symptoms vary depending on the amount and route of exposure. In 2021, journal Toxicon reported 50 cases of ricin intoxication leading to six deaths since 1980.
More of nature's deadliest species
