Cyanide

Cyanide was a white powder which was poisonous to many species. It killed by inhibiting cellular respiration. The Birastrop were not affected by it. (DW: The Brain of Morbius)

In 1899, Dr. Tiberius Finch used capsules filled with cyanide to compel his Blowfish creations. (TWN: The Baby Farmers)

In 1926, the Tenth Doctor was poisoned by Arnold Golightly with cyanide. Although by the time he noticed the symptoms of cyanide poisoning it would have been too late for a human to survive, the Doctor neutralised the poison by drinking ginger beer, eating walnuts for protein and anchovies for salt, and then getting a shock - a kiss from Donna Noble. This allowed him to eject the cyanide from his body through his mouth in a gaseous form. (DW: The Unicorn and the Wasp)

UNIT used cyanide gas to kill a swarm of giant cave insects. (DWAN: Caverns of Horror)

In the 1980s, Oscar Botcherby used cyanide to kill moths for his collection, saying it was quicker and kinder to them than other poisons. The Sixth Doctor used Oscar's cyanide to kill Shockeye of the Quawncing Grig. (DW: The Two Doctors)

The Fourth Doctor injected cyanide (in a gaseous form) into the laboratory of Mehendri Solon, killing him. Morbius survived as the body Solon had created for him included a set of lungs from a Birastrop. (DW: The Brain of Morbius)

When forced to create EDTA to cure arsenic poisoning, Gwen Cooper used cyanide in the antidote. (TW: Rendition)

Agatha Christie was an expert on poisons. (DW: The Unicorn and the Wasp)