See Carbogen: An Introduction for information about the health risks, dosage, and history of carbogen.

In the last nine years, I have administered carbogen to over fifty people, aged 22 to 65 years old. Nearly all of them were well-accustomed to profound psychedelic states of consciousness, but none had previously experienced carbogen. Most participated in multiple sessions each consisting of 2 to 26 inhalations. I have used three different concentrations: Meduna's mixture (30% CO₂ / 70% O₂) and two slight variations: 28/72 and 34/66. The 34/66 mixture produced a faster onset of effect, with no other noticeable differences.

All sessions involved pre-mixed gases in a single, high-pressure gas cylinder. For most sessions I used a pressure regulator and a mask with both a demand regulator and an extra exhaust valve. I added the extra exhaust valve to accommodate rapid exhalations (characteristic in carbogen subjects) that the standard valve couldn't handle. In a later setup I replaced the demand regulator with a bladder that was filled before the inhalations began. This allowed me to turn off the gas cylinder prior to carbogen administration, eliminating the risk of problems caused by open valves or a stuck demand regulator.

Before administering carbogen to others, I considered it essential to experience it myself. I tried carbogen twice to gain the first-hand experience necessary for administering it to others.

In my role as sitter, I answered any questions I could, but tried to avoid unduly influencing people's expectations. I warned subjects that the carbon dioxide in the mix could trigger an intense feeling of suffocation, but that they would not be in danger because they would be breathing more oxygen than is in ambient air. Sometimes, I used an analogy of an "idiot light" in the brain that checks for CO₂ concentration but ignores O₂ concentration. I also told them that the gas could taste pretty bad, almost like drinking a Coke and burping through the nose.

I asked each participant to select a maximum number of inhalations for their session, recommending between 10 and 25. I had participants lie down in a comfortable position and told them that I would count each breath out loud until I reached their chosen number. I told them I would then remove the mask, adding that if they made the slightest gesture during the session, I would immediately stop administering the gas. I mentioned that a new sensation could occur at around 7 or 8 breaths, replacing the initial discomfort, making it worthwhile to try to hold on through at least 10 inhalations. (One subject had a tremendously deep reaction to 2 breaths, so I agreed he could choose a low limit of 6 for his second attempt. At the sixth breath, when I removed the mask, he clutched at it, calling out "more, more . . . the angels!")

I did not measure pulse or blood pressure during sessions, as physiological responses are well described in Meduna's Carbon Dioxide Therapy. My primary goal was to gather information about the subjective mental state of the experience, but I will briefly describe carbogen's physical effects. Subjects generally exhibited one of three breathing patterns:

1. Deep and desperately rapid breathing starting by the second breath, and lasting into the teens. Somewhere before breath 20 the urgency tapers off and the subject settles into deep, rhythmic breaths.
2. Deep, rapid breathing with a gagging pause, as if to swallow, every 3 or 4 breaths into the 20s.
3. Calm, deep, slow breathing, through the 20s. One subject displayed this pattern during each session; another adopted it after his second session, and reported that it required quite a bit of will power.

Involuntary eye movements often started in the teens and lasted for 10 to 30 seconds after the last inhalation. If a subject's eyes were open during the session, they were apparently not focusing on anything external. None of the subjects I worked with experienced anything that I would classify as an "abreaction" (release of repressed subconscious material) as described in the literature from the 1950s and 1960s. When I removed the mask after the final breath, people consistently breathed hard for up to a minute, as though they had just run a short sprint.

In Their Own Words

After completing a session, many subjects attempted to put what they had experienced into words... After 26 breaths, subject uttered in a profound tone, "Death is very personal." He later described that he had experienced every cell in his body dying. He said that after the seventeenth inhalation he "could have gone on and on in that state." It was "very peaceful, very serene, kind of like postcards from heaven." Subject took 25 fast, deep breaths of carbogen with loud exhalations. His breathing quieted in about a minute and a half, and another minute later he exclaimed, surprised, "The mask's off? How many?" At his fourth inhalation he had seen "something coming on…colors." He and the other subjects in the room agreed, "It's essence-oriented". Subject took 13 breaths and then said, "Oh my, it was neat. I went to this paradise land. Wow. [I was in a] brightly starry zone, I drifted past that to lush, green paradise. […] After about 4 breaths I saw the stars and didn't think much about the breathing discomfort. […] It was very, very visual once I got past the star land. Then I was coming back and it's like, wait a minute! […] Similar to but totally different from DMT, even as far as the feeling of being just yanked." Subject took 24 inhalations and announced with a wide smile, "Oh, that was wonderful." Subject chose to take 13 inhalations, but stopped at 6, saying, "I can't imagine pushing it beyond that. It was incredible! Bright light, brighter than nitrous, and very alive. Impressive color." Subject found the entire experience uncomfortable, describing it as "dreadful". At 65 years old, this was the oldest subject. When asked to pick a number of inhalations, he blithely replied, "Why wouldn't I choose 25?". He emitted a low howl on every exhalation for about 30 seconds after the last breath. He described that it felt like he would suffocate and could not get over the discomfort. He said, "Glad I did it, but I'm not ready to do it again." He later explained that he had furiously resisted ether anesthesia as a child, and several doctors and nurses were required to hold him down before he went unconscious. Similarly, he fought his carbogen experience with all his will, in order to avoid giving up control. Subject took 10 inhalations, then giggled, "That was great." Subject took 25 breaths. During the experience, his body shook, seizure-like, then his left leg went up, then his right hand went into the air and he vocalized something unintelligible. After two minutes, his eyes changed to a look of recognition and his arm lowered and he said, "Wow, that was really intense! There was a whole blast of light and an interaction, that's why I had my hand up… a whole dream sequence like something out of Star Wars." Subject took 15 breaths and later wrote, "It was very different from my expectations. I barely even noticed the taste of the gas. I wasn't aware of any feeling of not having enough oxygen. I felt a surprisingly gentle sensation of drifting into a deep place of contentment, like I was floating down under the water, and colors were rising up to receive me. I was floating in a place outside of gravity, outside of boundedness."

As part of the protocol for the first U.S. group I worked with, I decided to direct potential subjects' questions about carbogen's effects to those who had already experienced it. I learned a great deal from listening to these accounts, and was surprised to hear sessions described in increasingly positive terms. The descriptions of ecstatic states were far beyond what I had expected.

I later conducted sessions with a large number of participants in the Netherlands. The setting was a dedicated spiritual space among "seekers" who had gathered for a psychoactives-oriented conference taking place in Amsterdam. I truncated the verbal part of my protocol for this group, since the subjects generally did not speak much English. They were asked to pick a number of breaths, but there was little discussion about expectations and potential effects. In this group, no subject stopped before their selected number of breaths. As with the U.S. group, the predominant reaction was "wow". There was no indication that any of these subjects experienced negative effects.

I hosted additional sessions in the United States, one with a small group of "drug geeks" curious about the effects of carbogen. They had researched and discussed carbogen with one another and had a sense of what they might expect. Individuals in this group were fairly conservative in the number of breaths they chose: between 5 and 22. Although I expected this group to choose higher numbers of inhalations, their collective opinion seemed to be that there was no clear benefit in going for more breaths; most thought that over 10 or 15 would not necessarily be more interesting or valuable.

Sometimes I saw what appeared to be terror or deep concern on people's faces, only to have them describe their experiences as wonderfully positive or at least neutral. For example, one subject's eyes shot open immediately after the mask was removed in what looked to observers to be an expression of fright or astonishment, yet he related an extremely positive, numinous mental state.

In early sessions, I felt it was best to fully disclose the potentially uncomfortable physical sensations often experienced in the first 10 breaths. After administering carbogen dozens of times, I found that most people did not have much trouble with the experience. Because of this, I felt it was less necessary to spend time describing possible negative reactions to later subjects. I wonder what the reactions would be if pre-session preparation focused on positive aspects of the carbogen experience.

Clearly, the profound experiences generated by psychedelics can be highly influenced by suggestion. It seems this is true for carbogen as well. Although some early authors reported frequent negative reactions to carbogen, in my sample of more than 50 subjects, only one exhibited an obvious negative reaction and none appeared to abreact. This may be because my subjects' previous work with psychedelics left them clear of psychic debris. Or they may simply have been accustomed to powerful psychoactive effects, and thus better prepared for the experience. Also, all of the subjects to whom I administered carbogen were specifically interested in having a carbogen experience rather than receiving it as a part of therapy or research. Finally, when carbogen is presented as an extraordinarily powerful but intrinsically neutral or positive experience, it appears to be less likely to produce a negative reaction.