A drug that produces the outward appearance of death, which McCoy injects into Kirk during his duel with Spock on Vulcan while claiming it is a tri-ox compound.
The neural paralyzer is used in the Original Series episode Amok Time. T'Pring's challenge forces Spock, deep in the blood fever, to fight Kirk to the death in the thin, hot air of Vulcan.
McCoy asks T'Pau's leave to give the captain an injection of tri-ox compound to help him breathe. The hypospray in fact holds a neural paralyzer, which takes effect in the middle of the combat: Kirk goes limp in Spock's grip, shows no life signs and is declared dead.
The ruse ends the ritual without either officer dying. Kirk revives aboard the Enterprise, and Spock's relief at finding him alive produces one of the character's rare open displays of emotion.