Tri-ox was a simple but effective intravenous medicine used for rapidly oxygenating the blood cells of a living being, especially in an oxygen-poor environment such as Vulcan. Most Humans and humanoids would otherwise have had trouble getting sufficient oxygen in such environments, especially during any strenuous work or other exertions. The drug could be used as a substitute for cumbersome breathing masks and equipment in such cases.
Tri-ox compound was designed to strengthen an individual's cardiopulmonary system. An injection of 15 ccs of tri-ox every four hours was the recommended dosage taken to compensate for excess carbon dioxide, as could be the case of a humanoid in prolonged exposure to a Class L environment. Without it, an individual would experience the effects of hypoxia and possibly carbon dioxide poisoning. (DS9: "The Sound of Her Voice")
Doctor Leonard McCoy feigned using tri-ox on Captain James T. Kirk in 2267 during a Vulcan combat ritual, and later used the drug on the captain after his space suit's air supply had nearly run out. The following year, McCoy actually administered tri-ox to Kirk after he had been retrieved from an interphase. (TOS: "Amok Time", "The Tholian Web"}