Oneirogenic penthrene mist was a type of anesthetic gas used by the Starfleet in the mid-23rd century.
In 2256, Commander Ellen Landry ordered the USS Discovery computer to dispense oneirogenic penthrene mist into Ripper's containment pen, so that she could remove a claw for study. The mist, though, proved ineffective on the tardigrade, despite sensor readings suggesting otherwise. (DIS: "The Butcher's Knife Cares Not for the Lamb's Cry")