The Online Encyclopedia and Dictionary







This article is about the dangerous substance. For the band see Poison.
The symbol traditionally used to label a poisonous substance.
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, usually referring to naturally produced substances that kill rapidly in small quantities, such as the bacterial proteins that cause tetanus and botulism. A distinction between the two terms is not always observed, even among scientists. Animal toxins 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 to defend itself while still alive. A single organism can be both venomous and poisonous.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), although since this attracts children (being linked to pirates) it is gradually being replaced by Mr. Yuk.


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.

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 with which they come in contact 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)

Metabolic poisons act by adversely disrupting the normal metabolism of an organism.

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:

  • beryllium (a highly but subtly toxic light metal)


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 .


  • Fasciculin

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)

A carcinogen is a chemical substance which is believed to cause cancer. There are an enormous variety of possible carcinogens. Some of the better known or more controversial examples are listed below.

  • Some artificial sweeteners (e.g. Aspartame and Saccharin) have been alleged to be carcinogenic or neurotoxic (however these research behind these claims is highly controversial and inconclusive; the FDA believes aspartame is safe for humans in dietary doses).
  • Asbestos - a widely used insulating material that causes mesothelioma and other cancers especially in the respiratory tract.
  • Benzene (lab solvent, used in various chemical reactions).
  • Carbon tetrachloride (formerly used in fire extinguishers).
  • Dioxin - actually a group of many chemicals - has carcinogenic and other toxic effects.

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
    • 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

See also victims of poisoning

Confirmed poisonings

Suspected or rumoured poisonings

Poisons in crime fiction

This list is incomplete, given that poisoning is a frequent plot twist in crime fiction.



See also

The contents of this article are licensed from under the GNU Free Documentation License. How to see transparent copy