Toxin

Toxins were substances that could be unhealthy or poisonous to certain organisms. Some could be harmful or even fatal to some species while others were not affected.

Due to human activity, Earth had a lot of toxins and dioxins in its atmosphere, which would have provided decent nutrition for the Nestene Consciousness. (TV: Rose)

The Urbankan ruler, Monarch, developed a toxin to release it on Earth in order to eliminate the residing humans and to replace them with his people. Monarch was not entirely immune to the toxin. When Monarch was about to fire on the Fifth Doctor, Adric threw the virus at Monarch and was shrunk down upon exposure to the virus. (TV: Four to Doomsday)

Duskflies had poisonous stingers, but the toxins in their stings could be neutralised by consuming a certain type of nut. (PROSE: You Shouldn't Have)