dad, do you have a cyanide pill?

"No!" Was the response Vale of RE/Search gave when asked by his 9 year old daughter. "Do you even know what a cyanide pill is?" "No." Said Valentine. I didn't really know either. I mean, I knew the Hollywood cyanide capsule, but there's way more to it than that

A cyanide is any chemical compound that contains the cyano group -C≡N, with the carbon atom triple-bonded to the nitrogen atom.

The CN group can be found in many kinds of compounds. Some are gases, others are solids or liquids. Some are salt-like, some covalent. Some are molecular, some ionic, and many are polymeric. Those that can release the cyanide ion CN− are highly toxic.

Hydrogen cyanide is a colorless gas with a faint, bitter, almond-like odor. Nearly 40 percent of the population is unable to smell hydrogen cyanide. This seems to be genetically determined in a complex fashion[1]. Sodium cyanide and potassium cyanide are both white powders with a bitter, almond-like odor in damp air, due to the presence of HCN formed by hydrolysis

Cyanides can be produced by certain bacteria, fungi, and algae, and are found in a number of foods and plants. In plants, cyanides are usually bound to sugar molecules in the form of cyanogenic glycosides and serve the plant as defense against herbivores. Cassava roots (aka manioc), an important potato-like food grown in tropical countries, reportedly contain cyanogenic glycosides[2][3]

The Fe-only and [NiFe]-hydrogenase enzymes contain cyanide ligands at their active sites. The biosynthesis of cyanide in the [NiFe]-hydrogenases proceeds from carbamoylphosphate, which converts to cysteinyl thiocyanate, the CN- donor.

"Cyanide" is a staple of crime fiction and publicly regarded as meaning deadly poison. Many cyanide-containing compounds are indeed highly toxic, but many are not. Prussian blue, nominally Fe7(CN)18, a common pigment, is administered orally to counteract the effects of poisoning by Thallium and 137Cs.

The most dangerous cyanides are hydrogen cyanide (HCN) and salts derived from it, such as potassium cyanide (KCN) and sodium cyanide (NaCN), but including others. Also some compounds readily release HCN or the cyanide ion, such as trimethylsilyl cyanide (CH3)3SiCN upon contact with water and cyanoacrylates upon pyrolysis. [citation needed]

Many thousands of organic compounds contain the CN group. These compounds are called nitriles. Generally, nitriles do not display the toxicity of HCN, NaCN, and KCN. In fact, the nitrile functional group is an integral component of numerous pharmaceutical drugs including cimetidine (Tagamet), verapamil (Isoptin), and citalopram (celexa). The reason for their diminished toxicity is that nitriles do not release the CN− ion, which permanently binds to and inhibits cytochrome c oxidase, the specific basis of the lethality of cyanide.

Cyanide ions bind to the iron atom of the enzyme cytochrome c oxidase (also known as aa3) in the fourth complex in the mitochondrial membrane in the mitochondria of cells. This deactivates the enzyme, and the final transport of electrons from cytochrome c oxidase to oxygen cannot be completed. As a result, the electron transport chain is disrupted, meaning that the cell can no longer aerobically produce ATP for energy.

Tissues that mainly depend on aerobic respiration, such as the central nervous system and the heart, are particularly affected.

Plants contain a cyanide-insensitive pathway (photosynthesis instead of the redox reaction) for respiration in their mitochondria, and as a result are insensitive to concentrations of cyanide that are lethal to animals.

from Wikipedia