Hallucinogen

Hallucinogens are a broad category of psychoactive substances that alter consciousness, leading to significant shifts in perception, mood, thinking, and the sense of self. They are sometimes called psychedelics, entheogens, or, historically, psychotomimetics. While often grouped into three main types—psychedelics, dissociatives, and deliriants—some hallucinogens don't fit neatly into any of these categories.

Examples of hallucinogens include psychedelics (like LSD, psilocybin, mescaline, and DMT), dissociatives (such as ketamine, PCP, DXM, and nitrous oxide), and deliriants (like scopolamine, diphenhydramine, and trihexyphenidyl). Others work through different mechanisms, including κ-opioid receptor agonists (salvinorin A, pentazocine, levorphanol), GABAA receptor agonists (muscimol, gaboxadol), oneirogens (ibogaine, harmaline), and cannabinoids (THC, nabilone, JWH-018). Additional substances like carbogen, hallucinogenic bolete mushrooms, and nutmeg are also considered hallucinogens.

To define a drug as hallucinogenic, Leo Hollister proposed five criteria: changes in thought, perception, and mood should dominate over other effects; memory or intellectual impairment should be minimal; stupor, narcosis, or excessive stimulation shouldn't be central; autonomic nervous system side effects should be minor; and addictive craving should be absent. Richard Glennon added two more criteria specifically for classical hallucinogens: they must bind to 5-HT2 serotonin receptors and be recognized by animals trained to discriminate the drug DOM.

Most hallucinogens are categorized by their pharmacology: psychedelics are serotonergic, dissociatives are generally antiglutamatergic, and deliriants are generally anticholinergic. However, some, like salvinorin A and ibogaine, don't fit these categories. Entactogens and cannabinoids are sometimes included as hallucinogens as well. The term "hallucinogen" can refer to this broad class or, in some contexts, only to classical hallucinogens (psychedelics), so it's important to check the definition used. Because the subjective effects are so varied, no standardized terminology based on experience has been widely accepted.

Classical hallucinogens have gone by many names. As David E. Nichols noted in 2004, earlier terms included "phantastica," while "hallucinogen," "psychotomimetic," and "psychedelic" have been used interchangeably. "Hallucinogen" is most common in scientific literature, though it's an inaccurate description of the actual effects. In the lay press, "psychedelic" remains popular. More recently, "entheogen" (from Greek, meaning "god within") has gained traction in counterculture and media, suggesting these substances connect users to a divine inner experience. Robin Carhart-Harris and Guy Goodwin argue that "psychedelic" is preferable to "hallucinogen" for classical psychedelics, since the latter misleadingly emphasizes hallucinations. Nichols disagrees, reserving "psychedelic" for serotonergic psychedelics specifically. Some hallucinogens are designer drugs, like those in the 2C and 25-NB (NBOMe) families, which are structural or functional analogs of controlled substances designed to mimic effects while avoiding legal classification or detection.

Different classes produce different effects. Psychedelics, or serotonergic classical hallucinogens, act as 5-HT2A receptor agonists. Despite attempts since the 19th and 20th centuries to define common patterns of experience, no universally accepted taxonomy for their effects exists.