Table 1 Glossary of key terms.

Classic Psychedelic

The term “psychedelic” was first coined by Dr. Humphrey Osmond in 1957 to describe the mind-altering effects of drugs that are thought to act primarily as agonists at the serotonin 2 A receptor (5-HT_2AR) (e.g., psilocybin, mescaline, LSD, DMT, but not ketamine). The word has since become synonymous with countercultural art and music, and a nonspecific catch-all term for hallucinogenic and visionary experiences, hence the addition of ‘Classic’ to denote this subset of 5-HT_2AR agonist hallucinogens.

Delusion

Delusions reflect changes in normal thinking processes, generally when a person can become very confident in spurious beliefs (e.g., people are plotting against them or they are receiving special messages from the world around them). In psychedelic experiences, changes in thinking may reflect paranoid or grandiose delusions that could be considered adverse events, though may also relate to experiences viewed as insightful or spiritual (e.g., ego-loss, or experiencing oneself as one with the universe).

Dissociative Anesthetic

A term first introduced in 1965 by Dr. Edward F. Domino and colleagues, coined by Domino’s wife Toni, to describe the dreamlike state induced by the drug ketamine. These drugs are primarily thought to act as glutamatergic NMDA-receptor antagonists.

Entactogen

A term first introduced by Dr. David E. Nichols in 1986 to describe the effects of MDMA and similar compounds that can amplify positive emotions, empathy, and social connectedness. These drugs are primarily though to act via serotonin, norepinephrine, and dopamine reuptake inhibition and release.

Hallucination

Hallucinations are false sense percepts without corresponding stimuli in the external world, which can exist across any sensory domain (i.e., vision, hearing, smell, taste, and touch / interoception). Elementary hallucinations comprise basic false perceptions like a taste of sweetness or a hum, whereas complex hallucinations can be far more elaborate like seeing entire landscapes of intricate scenery or hearing voices telling a story.

Hallucinogen

A pharmacologically diverse group of psychoactive drugs that can cause perceptual alterations (including hallucinations, pseudo-hallucinations, and illusions, defined below) and widely varied changes in thinking, emotion, and sense of self. As with all drugs, these effects may differ according to dose and route of administration.

Illusion

Illusions are defined as perceptual distortions or misinterpretations related to actual objects, such as seeing something as larger or smaller than it really is or perceiving a false sense of movement in stationary objects.

Ketamine

Ketamine is a dissociative anesthetic that is used widely as an anesthetic and at sub-anesthetic doses has also shown promise as a rapid-acting treatment for depression, suicidality, and substance use disorders (among others). Side effects may include drowsiness, nausea, disorientation, and increased blood pressure and heart rate.

MDMA

MDMA is an entactogen that has primarily been studied and shown effectiveness for the treatment of post-traumatic stress disorder (PTSD) in combination with structured therapy. Side effects may include increased blood pressure and heart rate, headache, and hyperthermia.

Pseudo-hallucination

Pseudo-hallucinations may constitute elements of both elementary and complex hallucinations, but with the accompanying insight that the perceptions are unreal or imagined, which is typically lacking in true hallucinations.

Psilocybin

Psilocybin is a naturally occurring classic psychedelic that has shown potential as a treatment for depression, anxiety, existential distress, and substance use disorders (among others), typically in combination with psychological support. Side effects may include increased blood pressure and heart rate, transient anxiety, panic or paranoia, nausea, and headache.

Psychonaut

Psychonaut refers to a person who endeavors to explore the mind or psyche, often via techniques such as ingesting psychoactive substances, meditation, or sensory deprivation.

Psychoplastogen

The term psychoplastogen was first coined by Dr. David E. Olson in 2018 to denote drugs that can promote rapid structural and functional neural plasticity such as growth of new synapses and dendrites between neurons. This psychoplastogenic property has been proposed as a biological mechanism underlying the rapid-acting therapeutic effects of various drugs including ketamine, MDMA, and psilocybin.

Synaesthesia

Synaesthesia is an intermingling of sensory signals when information from one sense elicits corresponding sensory experiences in another domain, for instance a sense of tasting colors, or seeing sounds.