Nerve gas

Nerve gas was a weaponised gas which attacked the nervous systems of those who came into contact with it. Humans who were affected by nerve gas were often put to sleep.

Thunderbolt was a British nuclear missile packed full of nerve gas. Before the start of the World Peace Conference, UNIT was put in charge of its disposal at the bottom of the ocean. This was interrupted by the reappearance of and a revolt at Stangmoor Prison, of which the Master took full charge. He planned to use escaped convicts to hijack the Thunderbolt and fire it at the Peace Conference thus starting World War III. The Third Doctor was able to activate its abortion mechanism, and UNIT destroyed the missile. (TV: The Mind of Evil)

The Zygons used what the Fourth Doctor characterised as "nerve gas" to knock out residents in and around the village of Tulloch. Doctor Harry Sullivan did not dispute this assessment. (TV: Terror of the Zygons)

Working on captured Zygons, Sullivan later developed a nerve gas called Z-67 which could unravel their DNA, essentially turning them inside-out. If released it would create a chain reaction in the atmosphere, such as that a single use would kill all Zygons on an entire planet. The Doctor, whose twelfth incarnation later called Sullivan an "imbecile" for creating it, confiscated the gas and the formula from UNIT. The red Osgood Box supposedly contained Z-67 that would be released if the humans chose the "winning" button. However, this was a feint by the Twelfth Doctor, and the box was empty. (TV: The Zygon Invasion, The Zygon Inversion)

Aboard Space Station Camera, the Sixth Doctor anticipated that jets of nerve gas would be vented in the event of a defence alert. (TV: The Two Doctors)

Within the Matrix, the Valeyard employed asphyxiating nerve gas against the Sixth Doctor and Sabalom Glitz. (TV: The Ultimate Foe)