Poisons were materials that, when ingested, could cause harm or kill an organism.
Poppaea, jealous of Barbara, ordered court poisoner Locusta to poison Barbara's drink. Vicki switched cups so that Nero's was poisoned, and informed the First Doctor of her actions. When the Doctor told Nero that his drink was poisoned, Nero gave it to his slave Tigilinus, who drank it and died. (TV: "Conspiracy")
The Macra fed upon a poisonous gas and had humans mine it for them. (TV: The Macra Terror) In the far future, a devolved variant of the species fed on the poisonous gases that accumulated underneath the Motorway. (TV: Gridlock)
The Zircon attempted to wipe out the Earth with a galactic cyclone, which would be in the state of a poison cloud by the time it arrived. The plan was stopped, however, by the Third Doctor and Sarah Jane Smith using a shield of static electricity. According to another account, the plan was thwarted by the Fourth Doctor and his companion Joan Brown. (COMIC: Doomcloud)
Cybermats infected the crew of Nerva Beacon with a poison through their bite. Warner was killed in this way and Sarah Jane was only cured after travelling through a transmat. (TV: Revenge of the Cybermen)
Dianene was a poison that took effect once it was inhaled. Megropolis One was fitted with a sprinkler system that could release dianene in the event of a crisis as a part of "Contingency Plan A". (TV: The Sun Makers)
The poison cyanide was used for a variety of purposes, such as killing individuals, (TV: The Brain of Morbius) killing moths for a personal collection, (TV: The Two Doctors) or creating antidotes. (TV: Rendition) Birastrop were immune to the effects of cyanide. (TV: The Brain of Morbius)
Coming into contact with raw spectrox would cause spectrox toxaemia and lead to death. The Fifth Doctor and Peri Brown both became infected while on Androzani Minor. The Doctor managed to save Peri with the milk from a queen bat but was forced to regenerate to save his own life. (TV: The Caves of Androzani)
The Judas tree contained a deadly poison which could stop a Time Lord from being able to regenerate. Trained by the Silence, Melody Pond kissed the Eleventh Doctor with lipstick made from the poison of the Judas Tree. He was only saved when she sacrificed her remaining regenerations to revive him. (TV: Let's Kill Hitler)
Jocrassa Fel-Fotch Passameer-Day Slitheen, disguised in the skin suit of Joseph Green, complaining that he was getting "poisoned by the gas exchange" after he farted. He relieved himself by disrobing. (TV: World War Three)
Behind the scenes
In the real world, such an infection as Sarah receives in Revenge of the Cybermen would likely be termed venom, but the serial explicitly uses the word poison.