Tri-ox is an intravenous medicine used to increase oxygen in a biological entities blood stream. It also strengthens a person's cardiopulmonary system allowing them to surface for extended periods of time in environments with a lower oxygen level that what is usual for that species.
A recommended dosage for Humans in a carbon dioxide rich environment such as a Class L planet is 15 ccs every four hours. While stranded on an L class world Lisa Cusak managed to survive for longer by taking decreased dosages. (DS9: "The Sound of Her Voice")