Hydrogen cyanide

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)

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)

Catherine's cat was poisoned by accidental exposure to prussic acid. (PROSE: Lackaday Express)

Hydrogen cyanide composed 9% of the 456 atmosphere. (TV: Children of Earth: Day Two)