"Carbogen" refers to a gaseous mixture of carbon dioxide (CO₂) and oxygen (O₂), most often administered via a mask attached to a regulator connected to a high-pressure cylinder. The ratio of 30% CO₂ and 70% O₂ is known as "Meduna's mixture", after Ladislas J. Meduna, a psychiatrist who pioneered its use as a therapeutic tool in the 1940s and 1950s. Although carbogen—sometimes described in the medical literature as a "panicogen"—is perhaps best known for inducing anxiety, sensations of suffocation, and unconsciousness, this extremely simple gas can cause surprisingly complex psychoactive effects when inhaled for even a few breaths. Reports of discomfort and anxiety experienced while breathing carbogen may be partially explained by the set and setting of traditional carbogen administration rather than properties inherent in the substance itself. Recent reports of positive, or even glowing experiences suggest carbogen may be more properly described as a unique psychedelic.

Although carbogen has been used safely for over 50 years, prolonged exposure to high levels of carbon dioxide can be fatal. Individuals with weakened respiratory health or any pulmonary disease are at much higher risk of potentially life-threatening responses. Additionally, those suffering from panic or psychotic disorders are more likely to experience adverse reactions (see Carbogen in Psychiatric Research below).

Carbogen doses are typically measured in number of breaths, with a fully immersive experience resulting from 8–20 breaths. The maximum number of breaths administered, even in extreme cases, seems to be 30–50. Inhaling more than 50 breaths of Meduna's mixture is generally considered unproductive and increasingly dangerous.

Most people remain relatively still for the duration of the experience. As few as 1–5 breaths can cause significant alterations of thought, perception, and mood; rushes of sensation in the body; and disorientation. During these first moments it may require a willful effort to continue inhaling the acidic-tasting gas. After a few more breaths, effects increase in intensity, and awareness of the external world fades away. People often close their eyes and those who keep them open exhibit an unseeing stare. Breath rate generally increases and breathing becomes somewhat labored.

When inhaled at sufficient doses, carbogen has anaesthetic and dissociative effects, frequently accompanied by both open- and closed-eye visuals. These effects include brightening or darkening of the visual field, kaleidoscopic or swirling changes in color, and geometric patterning.

Some people experience visions of more complex dreamscapes involving recognizable people and objects, or the sensation of being transported to another place or being in another dimension. Reports may include spiritual elements such as ineffable, numinous mental states, or encountering angelic beings.

Psychoactive effects continue to develop for a short period after the final breath. They then subside rapidly with subjects returning to near baseline within 45–180 seconds after the last inhalation. As the effects fade, the feeling of having experienced more than can be remembered is common. In another parallel with more traditional psychedelics, Meduna noted that a small number of his subjects vividly re-experienced insights or memories that surfaced in carbogen sessions hours later or even the following day.1

Although the cognitive component of the experience varies substantially from person to person and from experience to experience (as with many psychedelics), perhaps the most universally accurate descriptor is simply "intense".

Physical symptoms frequently include rapid, deep, or shallow breathing. Even though carbogen contains more oxygen than ambient air, its higher-than-normal levels of carbon dioxide trigger brain reflexes associated with asphyxiation (see Carbogen Pharmacology below).

Meduna reported other physical symptoms, such as eyelid fluttering and body spasms, but more recent carbogen reports do not support these as universal or even particularly common. At 10–30 breaths, some subjects flex their leg and hip muscles, or have carpal spasms (involuntary finger and wrist curling) similar to those sometimes experienced during Holotropic Breathwork. In intense sessions (40 breaths or more), Meduna reported that some subjects underwent convulsions, seizures, or bicycling of the limbs.1

When inhaling carbogen, it is common for subjects to feel that they are not getting enough oxygen. A recent experimenter said, "It's like feeling like suffocating".2 One of Meduna's patients reported: "In every case, when inhaling the gaseous mixture I experienced a terrifying smothering sensation; and before the moment of narcosis I had to exert every bit of my will power to keep from fighting the mask."1

These feelings of suffocation can cause mild to severe anxiety, fear, or panic, as described by another patient: "After three or four inhalations, the feeling of suffocation becomes intense. And then it becomes necessary to breathe fast. About this time I start praying, 'Oh, God, please help me to go under this quick,' and it usually helps. However, there are times when nothing helps, I am so filled with panic."1

In a similar spirit, Dale Pendell captures the flavor of a very unpleasant experience: "It's bad from the first lungful. It tastes sour. Alarm bells go off immediately. 'This is bad.' Will power is required to take even one full inhalation. By the second lungful, if you haven't ripped the mask off, full panic has set in. You need AIR! People start gasping at this point, faces flushed and sucking in the carbogen furiously."3

This feeling has also been described as generalized anxiety: "I got 'that scary feeling', the early tinges of the fear that I might get as I approach the difficult part of a [psychedelic] trip."4

Psychiatric research using carbogen has found that those who have anxiety or panic disorders are more prone to negative reactions than the general population. Some research has also shown that a majority of subjects experience at least some symptoms of increased anxiety during extended "carbon dioxide challenges".5 Meduna found that administering carbogen in shorter sessions (no more than 30 breaths) seemed to help. He observed that fear often grew more intense as a session progressed.

Descriptions of carbogen's unpleasantness abound. A 1992 book by Trevor Trueheart briefly mentions carbogen: "Everybody I have known to inhale carbogen experienced extreme fear and seemed to confront all the demons stored in his or her unconscious. This is the stuff of nightmares".6

In stark contrast to the many negative reactions are the extremely positive, sensual, or euphoric responses. In fact, Yacorzynski, et al. (1962) reported that more of their subjects "enjoyed" the experience than found it frightening. Recent work by B. James (see Carbogen Redux) involving subjects familiar with psychedelics confirmed that a majority of those who were administered carbogen in a friendly setting did not have fearful reactions, and many found the effects pleasurable. Giggling and laughing were common and "wow" was probably the most frequently uttered word after a person's first experience.

Immediately following his first experience with carbogen (15 breaths), one person took 4 breaths of normal air and then exclaimed excitedly: "Yeah, this is fucking awesome. This is it, I love this. Fuck, this is so cool! [breathing hard, laughing] That's wonderful shit. Excuse my language. [breathing calms] It didn't feel alien to any experience I've had; it seems akin to the more euphoric states of nitrous."4

Many people report enjoying the aesthetic effects or visions, as in these 1998 reports: "It was intensely beautiful."2 and "Oh my, it was neat. I went to this paradise land: wow! […] I went through a brightly starry zone, then drifted on past that to a lush, green paradise. It was neat: beauty. It was very good."2

Some subjects immediately comment that they'd like to try it again: "I sure went someplace that I've never been before. Yeah, I think that I'd love to go back."2 Others comment on the experience with superlatives: "I like that stuff. It could be the best."2

Positive psychedelic effects have also been reported by subjects administered carbogen in medical settings, such as this account from a session administered by Meduna:

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 designs, some geometric, some fanciful and graceful—purple and rose coloring predominant. As these sheets came towards me they seemed to engulf me and leave me breathless in the mad rushing sensation. Then the colors left and I felt myself being 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. Through this communion I seemed to receive assurance that the petit problems or whatever was bothering the human being that was me huddled down on the earth, would work out all right and that I had no need to worry.

In this spirituelle I felt the Greater Spirit even smiling indulgently upon me in my vain little efforts to carry on by myself and I pressed close the warmth and tender strength and felt assurance of enough power to overcome whatever lay ahead for me as a human being.1

Meduna commented that, "In this beautiful experience we can discern almost all the constants of the CO₂ experience: (1) Color; (2) Geometric pattern; (3) Movement; (4) Doubleness of personality; and (5) Divination or feeling of esoteric importance."1

The disparity between panicky, uncomfortable experiences and pleasurable, spiritual ones raises questions about what accounts for these differences. The bias towards negative descriptions may be partially the result of the fact that Meduna administered carbogen primarily to individuals seeking treatment for emotional or mental problems. It may also result from the expectations of researchers and participants.

Yacorzynski, et al. (1962) found that the majority of their subjects did not experience feelings of fear, speculating that was due to differences in the briefings given beforehand.7 After administering carbogen to dozens of participants, B. James came to believe that the quality and type of experience may be as strongly affected by methods, contexts, and expectations as they are with most psychedelics. Set and setting may be critical factors in the experience of carbogen. Psychiatric research into anxiety using carbogen confirms this, with a number of papers showing that a sense of safety, feeling in control of the administration of the gas, an understanding of carbogen's effects, and having a "professional" present during the session can all lead to reduced anxiety.8

Although the literature is dominated by descriptions of anxiety and discomfort, it may actually be that most people who breathe carbogen in an appropriate setting do not experience strong unpleasant effects.

Inhaled carbon dioxide has been known to have psychoactive effects since at least 1827, when Henry Hickman demonstrated that a dog exposed to an atmosphere containing carbon dioxide became anaesthetized.1 In 1856, the author Sir James Simpson stated that, "Carbon dioxide is recognized by toxicologists as a very powerful narcotic poison when inhaled in sufficient quantity."1 Carbon dioxide was explored as a medical and psychiatric tool in the late nineteenth and early twentieth centuries.

The golden age of carbogen research in psychiatry arrived in the 1940s and 1950s, spearheaded by the work of Ladislas Meduna. Born in 1896, Meduna was a Hungarian psychiatrist who emigrated to Chicago in 1938. He developed a deep interest in psychiatric therapies involving sleep, coma, and related non-ordinary states of consciousness and was an early pioneer in chemical and electroconvulsive therapies.

Meduna began researching carbogen therapy in 1943 and over the course of his career conducted more than 15,000 carbogen "treatments".1 His standard procedure consisted of administering between 10 and 50 inhalations of the 30% CO₂ / 70% O₂ mixture in short sessions, three times per week.

Meduna reported success in using carbogen to treat several "psychoneurotic conditions" including stuttering, anxiety, spastic colitis, frigidity, and homosexuality. He reported that carbogen frequently induced the surfacing of repressed emotional responses, which he considered unrelated to carbogen's therapeutic effects. Meduna discussed the possibility that the intensity of the experience, coupled with the brief unconsciousness it induced, might help desensitize patients to anxiety or fear, but largely considered carbogen's benefits to be a result of direct physiological action on the brain.1

In 1950 Meduna published the first edition of the definitive monograph on carbogen in psychiatry, Carbon Dioxide Therapy. In the 1950s carbon dioxide therapy research expanded and was used by other psychiatrists in conjunction with electroconvulsive therapy (also known as "electroshock therapy"). It was also co-administered with psychoactive drugs such as atropine, barbiturates, and antihistamines, although Meduna argued that combining carbogen with depressants could be dangerous.

While Meduna reported successes with carbogen, other researchers ultimately recommended against its use as an adjunct to psychotherapy. For example, Hargrove, et al. reported in 1954 that, "The use of carbon dioxide therapy in our hands added no specific therapeutic effect but did add problems of transference and resistance that retarded or prevented recovery."9

Carbogen use was investigated in the mid-twentieth century for a remarkable breadth of maladies ranging from depression, neuroses, psychoses, and dementias, to rheumatoid arthritis, cerebral palsy, Parkinson's disease, asthma, pneumonia, hiccups, and whooping cough.7,10,11 In recent years, several studies have indicated that carbogen may help make certain cancerous tumor cells more susceptible to radio- and chemotherapies.12 Carbon dioxide also continues to be used in medicine for its ability to stimulate breathing and cause vasodilation.

Interest in carbogen as a psychotherapeutic tool waned in the 1960s. It was then increasingly investigated—in "carbon dioxide challenges"—for its capacity to cause anxiety and panic rather than treat them. In contemporary psychiatric research, carbogen has been "extensively used"13 in this type of study at concentrations of 5–35% carbon dioxide. It is notable, however, that recent research does not use the strong doses administered by Meduna, instead delivering high concentrations for only brief periods. As Zvolensky, et al. describe in a 2001 review of this field of research: "5% CO₂ typically is administered steadily for 15 min, 20% CO₂ for 20–25 [sec], and 35% CO₂ in a single vital capacity breath."8

One typical research protocol, published in 2001, administered 5% CO₂ for twenty minutes to subjects lying in a "respiratory canopy" and rated them for symptoms of panic. They found that 52.0% of those who had previously been diagnosed with panic disorder had a panic reaction, while 8.8% of the "normal comparison subjects" exhibited symptoms of panic.5 Other researchers have found an even wider disparity.

Carbogen's capacity to trigger anxiety has been used to evaluate treatments for anxiety disorders as well as the etiology (source) of panic reactions. In one double-blind study, panic responses triggered by carbogen were dramatically reduced in subjects with panic disorder by pre-treatment with 1 mg alprazolam (Xanax).5 Several carbogen studies have also shown that the closer one's familial relationship to a panic disorder sufferer, the more likely one is to experience panic during a carbon dioxide challenge.14

Alfred "Al" Hubbard, an important early advocate of psychedelics, appears to have been one of the first major proponents of carbogen within the psychedelic subculture. Hubbard traveled the world introducing famous and influential people to LSD and carbogen. As Jay Stevens describes in Storming Heaven, "One of Al's favorite break-the-ice devices was carbogen, a mixture of carbon dioxide and oxygen, which came in a small portable cylinder. Carbogen was what therapists referred to as a potent abreactor: ten or fifteen lungfuls and you tended to relive your childhood traumas. And judging on how well you handled them, Al would either offer to run an LSD session for you, or he wouldn't."15 Carbogen was, literally, Hubbard's (pre-) acid test.

Aldous Huxley, who received his first dose of LSD from Hubbard, recounts in Heaven and Hell:

[…] carbon dioxide produces, in those who inhale it, certain physical and psychological changes, which have been exhaustively described by Meduna. Among these changes the most important […] is a marked enhancement of the ability to 'see things,' when the eyes are closed. In some cases only swirls of patterned color are seen. In others there may be vivid recalls of past experiences. (Hence the value of CO₂ as a therapeutic agent.) In yet other cases carbon dioxide transports the subject to the Other World at the antipodes of his everyday consciousness, and he enjoys very briefly visionary experiences entirely unconnected with his own personal history or with the problems of the human race in general.16

Huxley and others speculated that spiritual practices involving the breath such as chanting or yogic breathing may induce religious visions through altering CO₂ concentrations in the blood.16 Stanislav Grof, a well-known researcher into LSD's psychotherapeutic effects, and Christina Grof later developed Holotropic Breathwork, a method of hyperventilation designed to legally mimic some of the beneficial effects of LSD. Holotropic Breathwork also produces its effects by directly manipulating CO₂ levels, though by decreasing rather than increasing CO₂ concentration in the blood.

Carbon dioxide and oxygen are readily available as compressed gases; mixtures of CO2 and O2 are sold in cylinders for various industrial, farming, and grocery uses.

Hubbard claimed to have conducted 1,700 LSD sessions by 1959, and his reputation piqued the interest of Myron Stolaroff, who went on to become an important figure in research into non-ordinary states of consciousness. Stolaroff writes, "Hubbard made a deeper impression on me than anyone I had ever met. It was an impression that radically altered my whole value-belief system, and completely changed the course of my life."17 The first day they met, Hubbard gave Stolaroff a tablet of methedrine and a few inhalations of Meduna's mixture. "No doubt reading me quite accurately," Stolaroff recalls, "he gave me only a few breaths, but it was enough to give me a glimpse, and especially a feeling, of another world."17 Stolaroff was profoundly impressed, and they arranged an LSD session.

Stolaroff's LSD session was life-changing. Convinced of its therapeutic potential, he set up the International Foundation for Advanced Study in Menlo Park, California in 1961. Stolaroff continued to work with carbogen: "In our Foundation work with clients, we found that several treatments with [Meduna's] mixture was an excellent procedure to introduce novices to altered states of consciousness. Many discovered unconscious contents of their mind for the first time. The procedure also cleared away a good deal of repressed material, thus freeing the subject for a smoother, more profound psychedelic experience."17 Although he considered carbogen beneficial, he noted "[…] it was such a drastic and dramatic procedure that no one underwent it eagerly."17

As a counterpoint, however, Stolaroff also describes a person who had a positive reaction to carbogen as "a complete standout. […] Jeffrey was quite an exception. He loved it. He would take an enormous number of breaths—up to 50".17 Such positive experiences were considered distant outliers. As legal psychedelic research became less common, interest in this potentially dysphoric psychoactive outside of research contexts was minimal, and carbogen experimentation largely disappeared.

The mechanisms of action for carbogen's psychoactive effects have not been fully established. During the 1950s and 1960s, the primary mechanisms proposed were alterations in blood pH and inhibition of neuronal function by the slowing of cellular metabolism. More recent research paints a complex picture of interconnected systems, including activation of serotonin (5-HT) neurons in the raphe nuclei (located in the mid-brain) where most serotonin neurons have their cell bodies.18,19,20

The air we breathe normally contains 20–21% oxygen and 0.02–0.06% carbon dioxide, with nitrogen and trace gases constituting the other 78.9+%. Meduna's mixture is 30% CO₂ and 70% O₂, giving it much higher concentrations of both than normal air. Although some people have reported mildly calming psychoactive effects from breathing pure oxygen, it is the extremely high levels of carbon dioxide (around 1000 times normal levels) to which carbogen's effects are attributed.

Counterintuitively, the physiological urge to breathe is mostly a response to high levels of CO₂ rather than to low levels of oxygen in the blood. While the body has some oxygen-based triggers, these account for only a small portion of the impulse to breathe. The carbon dioxide–driven breathing reflex explains why taking a lungful of 100% helium or nitrous oxide does not immediately cause the sensation of suffocation. In humans, central and peripheral chemoreceptors (chemical "sensors" inside and outside the brain) constantly monitor carbon dioxide levels in the blood to determine how strongly to breathe. Cells produce carbon dioxide as they use energy. When a person holds his or her breath, carbon dioxide levels rise in the blood and the chemoreceptors send "breathe more" alerts to the body and brain with increasing urgency. Carbogen triggers this response even though oxygen levels in the lungs and blood are higher than normal.21 Simply holding one's breath does not cause carbogen-like effects.

Carbon dioxide dissolved in blood is acidic, just as it is in carbonated soda. Inhalation of carbogen rapidly increases acidity (lowers pH) of the blood, cerebrospinal fluid, and brain, creating a condition called respiratory acidosis or hypercapnia. In 1938 Gibbs and Gibbs repeatedly measured arterial and venous blood pH while administering a 35-breath session with Meduna's mixture. They found that arterial blood pH dropped from 7.37 at the start of the session to 6.94 after 35 inhalations,17 a level that could be dangerous if maintained for prolonged periods. Blood pH rapidly returns to normal after carbogen administration ends. Although many researchers believe that lowered pH directly causes reduced brain activity and anaesthesia,22,23 acidosis alone probably does not explain carbogen's visual and visionary effects.

Interestingly, high levels of carbon dioxide also lead to rapid dilatation of cerebral vessels and increased cerebral blood flow.23,24 For brief periods, increases in blood flow in the brain are generally associated with improvements in cognitive performance.

Recent advances in understanding the identity and location of the brain's carbon dioxide chemoreceptors may provide better explanations for the anxiogenic and visionary effects of carbogen. Over the last decade, a number of researchers have verified that a subset of serotonin neurons in the raphe nuclei respond to hypercapnia.18 Because of the many brain areas to which they extend, such serotonin neurons are well-placed to not only trigger autonomic responses, such as breath and cerebral blood flow, but also interact with the parts of the brain responsible for cognition and emotions.19 Severson, et al. proposed in 2003, "that a major role of serotonergic neurons throughout the brainstem is to monitor the acid/base status of blood and to initiate multiple responses aimed at restoring pH to normal, including changes in behavior, breathing, and autonomic control."19 The highly-networked serotonin neurons—better known for their involvement with psychedelics, antidepressants, and higher brain function—are also valuable as CO₂ detectors because of their wide connection to many brain systems. When high CO₂ levels are detected, one might not only need to change breathing or increase heart rate, but may also need to be able to make complex, life-preserving decisions about how to get to a safer location.18,19,25

Ongoing research into the neurophysiology of anxiety and panic disorder has revealed that carbogen-activated chemoreceptor neurons may directly affect many parts of the brain, triggering several receptor systems, including the adrenergic, cholinergic, and dopaminergic systems. Kaye, et al. (2004) summarize the results of recent brain imaging research showing hypercapnia's widespread effects: "humans exposed to CO₂ have shown activation of numerous centres, including medullary, limbic and paralimbic (amygdala ad peri-amygdala), cerebellar, insula and prefrontal cortical areas."26,27

Carbogen psychopharmacology is still quite speculative and continues to be actively explored. Although there are thousands of papers listed in PubMed looking at the physiological and mental effects of increased carbon dioxide in the blood, including hundreds from the last few years, as far as we know, as of May 2007, the mechanisms of carbogen's psychedelic-like effects have not been examined in the scientific literature since the 1950s.

Carbogen mixtures have been safely administered in medical and psychiatric procedures many thousands of times. While it appears to be relatively safe when administered in brief sessions, prolonged exposure to high levels of CO₂ is sufficient to cause lethal respiratory or circulatory failure, as evidenced by the fact that CO₂ is used to euthanize lab animals.28,29,30

Hypercapnia is initially stimulating, but longer-term exposure to high levels of carbon dioxide leads to "carbon dioxide narcosis": lethargy, confusion, and sleepiness. Exposure to levels of carbon dioxide around 10% causes dizziness and higher levels cause unconsciousness within minutes.1 There have been a number of reported deaths where accidental exposure to high carbon dioxide levels caused the victims to black out before they could reach safety.28

Complicating the safety picture is the fact that most real-world cases involve not only high levels of carbon dioxide, but hypoxia (low levels of oxygen) as well. Most information provided by governmental agencies about CO₂ dangers is related to oxygen displacement. Serious accidents have been reported related to: dry ice (frozen CO₂) sublimating into CO₂ gas, beer brewing forming CO₂, and malfunctioning ventilation systems increasing CO₂ levels in submarines, among others. All of these involve hypoxia and are not exclusively attributable to CO₂ poisoning.

As Gill, et al. clarify in their 2002 article, it is important to distinguish between "simple asphyxiants" and "toxic gases". For example, nitrogen is a simple asphyxiant, meaning that if it displaces too much oxygen, it can cause suffocation, but as long as around 20% oxygen is maintained, it is safe to breathe indefinitely. Other gases, like carbon monoxide, hydrogen sulfide, or carbon dioxide can be fatal because of their chemical effects alone.28

Research with animals has convincingly shown that adequate concentrations of oxygen do not prevent death from CO₂ poisoning. One research protocol administered 80% CO₂ and 20% O₂ to dogs, causing death in 10–15 minutes. The same researchers also euthanized dogs with 50% CO₂ and 50% O₂, causing death in 30–90 minutes.29 Because of differences in metabolism as well as the long durations and high concentrations used, the animal research into CO₂ poisoning is not easily translatable to human health concerns for short-term inhalation of carbogen.

A primary danger of short-term inhalation of carbogen is accidental over-exposure, like the tragic but recurring unsupervised nitrous oxide suffocations caused by masks, garbage bags, or stuck tank valves. People with weakened respiratory systems, such as those with chronic obstructive pulmonary disease or cystic fibrosis, are at much higher risk of life-threatening complications from inhaling carbogen. Further, many heavy smokers have undiagnosed lung dysfunction that would severely weaken their ability to recover from acidosis caused by carbogen, putting them at much higher risk of respiratory failure than healthy individuals.31 Because carbogen inhalation causes a spike in cerebral blood flow, those with weak cardiovascular systems or health issues related to cranial blood pressure may be at higher risk of problems. Other populations may also be at higher risk, including schizophrenics or those suffering from anemia.

The psychoactive effects produced by a mixture of the common gases carbon dioxide and oxygen remind us that consciousness is a fragile knitwork of body, mind, and environment. Alexander Shulgin, the renowned psychedelic chemist, describes teaching a class in San Francisco after which a young man expressed interest in synthesizing LSD. Dr. Shulgin replied that LSD chemistry is very complex and that he should start off with something simple, like carbon dioxide. When the young man replied incredulously, Dr. Shulgin led him downstairs to a lab where tubs of dry ice were stored and suggested the student put his head down next to one of the tubs, inhale deeply through his mouth, and see how he felt. After taking two deep breaths and experiencing the psychoactive effects, he said, "Point taken".