Nitrous oxide
From Memory Alpha, the free Star Trek reference.
Nitrous oxide (N2O) is a colorless gas that often makes Humans insensible to pain and causes spontaneous laughter; due to this effect, it is often dubbed laughing gas.
When the computer circuits of the USS Enterprise were affected by subatomic particles from an energy field encountered by the ship and began playing practical jokes on the crew, among its tricks was to fill the ship with nitrous oxide.
After a few minutes of exposure to nitrous oxide, a Vulcan will pass out, as their physiology is unable to process the effects of the gas. (TAS: "The Practical Joker")
