From Bvio.com
The skull and crossbones symbol traditionally used to label a poisonous substance.

In the context of biology, poisons are substances that cause injury, illness, or death to organisms, usually by chemical reaction or other activity on the molecular scale. Some poisons are also toxins, and a distinction between the two terms is not always observed, even among scientists. The derivative forms "toxic" and "poisonous" are synonymous. Within chemistry and physics, a poison is a substance that obstructs or inhibits a reaction, for example by binding to a catalyst. Poisons have been known to be symbolized by the skull and crossbones (shown beside).

Biological poisoning

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 almost instantly starves the body of energy by poisoning mitochondria and the synthesis of 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. Such rapid reactions are often called acute poisoning.

A poison may also act slowly. This is known as chronic poisoning and is most common for poisons that bioaccumulate. Examples of these types of poisons are mercury, and lead.

Many substances regarded as poisons are toxic only indirectly. An example is "wood alcohol" or methanol, which is not poisonous itself, but is chemically converted to toxic formaldehyde 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 one individual and the next.

Milkweeds (Asclepias sp.) produce glycosides which are toxic to most organisms. (It is unlikely that any organism will eat much because of the extremely bitter taste.) Monarch butterfly larvae, however, are not susceptible to the toxin; in fact they accumulate it in their bodies as they eat the leaves of their host plant. Any predator who wishes to make a meal of an adult monarch will tend to be dissuaded by the bitter taste, and learn to leave the brightly colored insects alone. This gives considerable protection for monarchs from birds and other predators.

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.

Classification of biological poisons by mechanism


Corrosives mechanically damage biological systems on contact. Both the sensation and injury caused by contact with a corrosive resembles a burn injury.


Strong inorganic acids, such as concentrated sulfuric acid, nitric acid or hydrochloric acid, destroy any biological tissue they come in contact with within seconds.


Strong inorganic bases, such as lye, gradually dissolve skin on contact but can cause serious damage to eyes or mucous membranes much more rapidly. Ammonia is a far weaker base than lye, but has the distinction of being a gas and thus may more easily come into contact with the sensitive mucous membranes of the respiratory system. Quicklime, which has household uses, is a particularly common cause of poisoning. Some of the light metals, if handled carelessly, can not only cause thermal burns, but also produce very strongly basic solutions in sweat.


Poisons of this class are generally not very harmful to higher life forms such as humans, but lethal to microorganisms such as bacteria. Typical examples are ozone and chlorine, either of which is added nearly every municipal water supply in order to kill any harmful microorganisms present. All halogens are strong oxidizing agents, fluorine being the strongest of all.

See also: Free radical

Reducing agents

The most notable substance in this class is phosphorus.

Metabolic poisons (energy)

Specific biochemical inhibitors

Heavy metals

A common trait shared by heavy metals is the chronic nature of their toxicity. Low levels of heavy metal salts ingested over time accumulate in the body until toxic levels are reached.

Heavy metals are generally far more toxic when ingested in the form of soluble salts than in elemental form. For example, metallic mercury passes through the human digestive tract without interaction and is commonly used in dental fillings—even though mercury salts and inhaled mercury vapor are highly toxic.

Notable examples:


Neurotoxins interfere with nervous system functions and often lead to near-instant paralysis followed by rapid death. They include most spider and snake venoms, as well as many modern chemical weapons. One class of toxins of interest to neurochemical researchers are the various cone snail toxins known as conotoxins.


Acetylcholine Antagonists

Cell Membrane Disrupters


  • Nicotine - not strictly a neurotoxin, but capable in large doses of causing heart attack

Teratogens (birth defects)

Mutagens (DNA damage)

Carcinogens (cancer)

  • Carbon tetrachloride (formerly used in fire extinguishers)
  • Benzene (lab solvent, used in synthesis of various things)
  • Some artificial sweeteners have been alleged to be carcinogenic. ex. Aspartame, Saccharin
  • Dioxin - actually a group of many chemicals - has carcinogenic and other effects.
  • Asbestos - a widely used insulating material that causes mesothelioma and other cancers

Examples of biological poisons by source

Unfinished task: Items below should be added as examples under the appropriate poison class above.

Non-radioactive inorganic poisons

  • Acids and bases, corrosives
    • ammonia
    • The 6 strong acids are all listed above (besides HBr and HI, which are debatably important)
    • Lye
    • Lime, Quicklime, various light metal oxides, hydroxides, superoxides
    • Bleach, some pool chemicals, other hypochlorates (acidic and oxydizing effect)
    • hydrofluoric acid

Organic poisons

Naturally produced poisons and toxins

Famous poisonings

Poisons in crime fiction

This is of course an inexhaustive list. You may wish to add other works and/or specify the poisons used.



See also

Poison is also the name of a US rock band active in the 1980s and 1990s. For more information, see Poison.

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