A Starfleet medicine injected to help a patient cope with a thin or oxygen-poor atmosphere, used by McCoy as a cover story during Kirk's duel on Vulcan in Amok Time.
Tri-ox compound is a drug that helps the body make better use of what oxygen is available, and is given to people who must exert themselves in thin air.
It is best remembered from the Original Series episode Amok Time. When Kirk is forced to fight Spock in the heat and thin atmosphere of Vulcan, McCoy asks T'Pau for leave to give the captain a tri-ox injection so that the contest will be fairer.
What McCoy actually administers is a neural paralyser. It takes effect during the fight and makes Kirk appear to die, which ends the challenge, satisfies Vulcan custom and shocks Spock out of the blood fever. Kirk is revived aboard the Enterprise. Tri-ox itself continues to be mentioned as a standard remedy in later Star Trek series.