Nerve gas
From Tardis Wiki, the free Doctor Who reference
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.
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)
