Hydrogen cyanide, also known as HCN and prussic acid, was a chemical compound.
On the Sense Sphere, the First Doctor used a syringe of prussic acid to kill a Zilgan. (PROSE: The Monsters from Earth [+]The Dr Who Annual 1966 (Doctor Who annual, 1965).)
Mehendri Solon kept Morbius' brain alive using hydrogen cyanide as a colloidal nutrient. The Fourth Doctor used hydrogen cyanide to create cyanide gas in an attempt to kill Solon and Morbius. (TV: The Brain of Morbius [+]Robin Bland, Doctor Who season 13 (BBC1, 1976).)
Catherine's cat was poisoned by accidental exposure to prussic acid. (PROSE: Lackaday Express [+]Paul Cornell, Decalog (Virgin Decalogs, 1994).)
Hydrogen cyanide composed 9% of the 456 atmosphere. (TV: Children of Earth: Day Two [+]John Fay, Torchwood series 3 (BBC One, 2009).)
Zyklon B was a form of hydrogen cyanide. (AUDIO: The Nemonite Invasion [+]David Roden, New Series Adventures Audio (BBC Audio, 2009).)