Plants that cause illness or death after consuming them are referred to as poisonous plants. The toxins in poisonous plants affect herbivores, and deter them from consuming the plants. Plants cannot move to escape their predators, so they must have other means of protecting themselves from herbivorous animals. Some plants have physical defenses such as thorns, spines and prickles, but by far the most common type of protection is chemical.Keddy, P.A. 2007. Plants and Vegetation: Origins, Processes, Consequences. Cambridge University Press, Cambridge, UK, Chapter 7.
Over millennia, through the process of natural selection, plants have evolved the means to produce a vast and complicated array of chemical compounds to deter herbivores. Tannin, for example, is a defensive compound that emerged relatively early in the evolutionary history of plants, while more complex molecules such as polyacetylenes are found in younger groups of plants such as the Asterales. Many of the known plant defense compounds primarily defend against consumption by insects, though other animals, including humans, that consume such plants may also experience negative effects, ranging from mild discomfort to death.
Many of these poisonous compounds also have important medicinal benefits.Lewis, W.H. and M.P.F. Elvin-Lewis. 1977. Medical Botany. Plants Affecting Man's Health. Wiley, New York. 515 p. pp. 123–124. The varieties of phytochemical defenses in plants are so numerous that many questions about them remain unanswered, including:
Which plants have which types of defense? Which herbivores, specifically, are the plants defended against? What chemical structures and mechanisms of toxicity are involved in the compounds that provide defense? What are the potential medical uses of these compounds?
These questions and others constitute an active area of research in modern botany, with important implications for understanding plant evolution and medical science.
Below is an extensive, if incomplete, list of plants containing one or more poisonous parts that pose a serious risk of illness, injury, or death to humans or domestic animals. There is significant overlap between plants considered poisonous and those with psychotropic properties, some of which are toxic enough to present serious health risks at recreational doses. There is a distinction between plants that are poisonous because they naturally produce dangerous phytochemicals, and those that may become dangerous for other reasons, including but not limited to infection by bacterial, viral, or fungal parasites; the uptake of toxic compounds through contaminated soil or groundwater; and/or the ordinary processes of decay after the plant has died; this list deals exclusively with plants that produce phytochemicals. Many plants, such as peanuts, produce compounds that are only dangerous to people who have developed an allergic reaction to them, and with a few exceptions, those plants are not included here (see list of allergens instead). Despite the wide variety of plants considered poisonous, human fatalities caused by poisonous plants – especially resulting from accidental ingestion – are rare in the developed world.Krenzelok EP, Mrvos R.,"Friends and foes in the plant world: A profile of plant ingestions and fatalities." Clin Toxicol. 2011 Mar;49(3):142–149
Poisonous plants used as food Many plants commonly used as food possess toxic parts, are toxic unless processed, or are toxic at certain stages of their lives. Some only pose a serious threat to certain animals (such as cats, dogs, or livestock) or certain types of people (such as infants, the elderly, or the immunocompromised). Most of these food plants are safe for the average adult to eat in modest quantities. Notable examples include:
Scientific name Common name(s) Family Description PictureAleurites moluccanuscandlenut, Indian walnut, or kukuiEuphorbiaceaeThe seeds contain saponin, phorbol, and toxalbumins, they are mildly toxic when raw, inducing a laxative effect. Heat treatment reduces the toxicity of the protein component. Allium spp.| onion, garlic, leek, and chive | Amaryllidaceae | Many members of the genus Allium contain thiosulphate, which in high doses is toxic to dogs, cats, and some types of livestock. Cats are more sensitive.Cope RB. Toxicology Brief: Allium species poisoning in dogs and cats. Veterinary Medicine 2005
| Asparagus spp.| asparagus | Asparagaceae | Several species including Asparagus officinalis and Asparagus densiflorus. Though asparagus plants cultivated for food are typically harvested before they reach reproductive maturity, the berries of the mature plant are poisonous, containing furostanol and spirostanol saponins. Rapid ingestion of more than five to seven ripe berries can induce abdominal pain and vomiting. Sulfur compounds in the young shoots are also considered at least partially responsible for mild skin reactions in some people who handle the plant.
| Blighia sapida ackee Sapindaceae The seeds, unripened arils, and inedible portions of the ackee fruit contain the toxins hypoglycin A and hypoglycin B, which inhibit enzymes involved in fatty acid metabolism and thereby cause a depletion of stored glucose, which is necessary to meet the body's energy needs, and in turn leads to hypoglycemia. Citrus spp.| lemon, lime, grapefruit, orange, etc. | Rutaceae | Most citrus fruits, including lemon (Citrus limon), lime, and orange (Citrus × sinensis), among others, are known to contain aromatic oils, the terpenes limonene and linalool, and the furanocoumarin psoralen, which are toxic to dogs, cats, and other animals. The concentration and distribution of these compounds can vary widely by species, but they are often concentrated in the fleshy rinds. Symptoms include vomiting, diarrhea, depression, and photosensitivity. Citrus essential oils do not appear to be toxic to humans.