Cyanide

Cyanide was a white powder. It was poisonous to many species, though not to Birastrop (DW: The Brain of Morbius) or to Gallifreyans. (DW: The Unicorn and the Wasp)

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

In 1926, The Doctor was poisoned by Arnold Golightly using cyanide. The Doctor was able to avert dying by drinking ginger beer, eating walnuts and anchovies (containing salt), and then getting a shock, namely a kiss from Donna Noble. This ejected the cyanide from his body out his mouth in gas-like form. (DW: The Unicorn and the Wasp)

UNIT used cyanide gas to kill a swarm of giant cave insects. (DWA: 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 Doctor used Oscar's cyanide to kill Shockeye of the Quawncing Grig. (DW: The Two Doctors)

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