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Cyanides are chemicals that contain the cyano-group. In that group a carbon atom has three chemical bonds to a nitrogen atom. This group is present in many substances. Many of them are gases, but some are solids or liquids. Those substances that can release the compound CN are highly poisonous.
In popular culture, cyanides are said to be highly toxic. But there are many cyanides that really are, but many others that are not. Prussian blue, a cyanide compound, is given as a treatment to poisoning with Thallium and Caesium, for example.
The poisons referred to are usually hydrogen cyanide (HCN), and the chemicals which are similar to it, like potassium cyanide (KCN), and sodium cyanide (NaCN). (Such substances are called derivatives of hydrogen cyanide). Organic compounds that contain the CN group are called nitriles. Many of them are not as toxic. Some of them are even used to produce drugs.