Carbogen, also called Meduna's Mixture after its inventor Ladislas Meduna, is a mixture of carbon dioxide and oxygen gas. Meduna's original formula was 30% CO2 and 70% oxygen, but the term carbogen can refer to any mixture of these two gases, from 1.5% to 50% CO2.
When carbogen is inhaled, the increased level of carbon dioxide causes a perception, both psychological and physiological, of suffocation because the brain interprets an increase in blood carbon dioxide as a decrease in oxygen level, which would generally be the case under natural circumstances. Inhalation of carbogen causes the body to react as if it were not receiving sufficient oxygen: breathing quickens and deepens, heart rate increases, and cells release alkaline buffering agents to remove carbonic acid from the bloodstream.
Carbogen was once used in psychology and psychedelic psychotherapy to determine how a patient would react to an altered state of consciousness or to a sensation of loss of control. Individuals who reacted especially negatively to carbogen were generally not administered other psychotherapeutic drugs for fear of similar reactions. Meduna administered carbogen to his patients to induce abreaction, which, with proper preparation and administration, he found could help clients become free of their neuroses. Carbogen users are said to have discovered unconscious contents of their mind, with the experience clearing away repressed material and freeing the subject for a smoother, more profound psychedelic experience.
One subject reported: "After the second breath came an onrush of color, first a predominant sheet of beautiful rosy-red, following which came successive sheets of brilliant color and design, some geometric, some fanciful and graceful …. Then the colors separated; my soul drawing apart from the physical being, was drawn upward seemingly to leave the earth and to go upward where it reached a greater Spirit with Whom there was a communion, producing a remarkable, new relaxation and deep security."
Carbogen is rarely used in therapy anymore, largely due to the decline in psychotherapeutics.
Modern uses 
A carbogen mixture of 95% oxygen and 5% carbon dioxide can be used as part of the early treatment of central retinal artery occlusion.
Carbogen is used in biology research to study in vivo oxygen and carbon dioxide flows, as well as to oxygenate the ACSF solution and stabilize the pH to about 7.4 in research on acute brain slices.
Its use in combination with nicotinamide is also being investigated in conjunction with radiation therapy in the treatment strategy of certain cancers. Because increased tumor oxygenation improves the cell-killing effects of radiation, it is thought that the inhalation of these agents during radiation therapy could increase its effectiveness.
- Prisman E, Slessarev M, Azami T, Nayot D, Milosevic M, and Fisher J. (2007). Modified oxygen mask to induce target levels of hyperoxia and hypercarbia during radiotherapy: a more effective alternative to carbogen. International Journal of Radiation Biology. Jul;83(7):457-62.
- Erowid Carbogen Vault: Basics.
- Walsh, Roger N.; Charles S. Grob (2005). Higher Wisdom: Eminent Elders Explore the Continuing Impact of Psychedelics. State University of New York Press. pp. 57–58; 98. ISBN 0-7914-6517-9.
- Markoff, John (2005). What the Dormouse Said. Penguin. pp. 25, 62. ISBN 0-670-03382-0.
- Meduna, Ladislas J. (1950). Carbon Dioxide Therapy. Charles Thomas.
- Arnold JF, Kotas M, Fidler F, Pracht ED, Flentje M, Jakob PM. (2007). Quantitative regional oxygen transfer imaging of the human lung. Journal of Magnetic Resonance Imaging. Aug 8;26(3):637-645