Hallucinogens are a broad group of substances that alter perception, mood, and thought. They fall into three main categories: classic hallucinogens like LSD and psilocybin mushrooms, dissociative drugs like ketamine and PCP, and deliriants like scopolamine. Each category works through a different mechanism in the brain and produces a distinct type of experience.
Classic Hallucinogens
Classic hallucinogens produce their effects primarily by activating serotonin receptors in the brain, specifically a receptor involved in perception and cognition. This activation changes how sensory information is processed, particularly in the visual cortex, strengthening the brain’s “top-down” influence on what you perceive. The result is vivid alterations in sight, sound, and thought that can range from subtle color shifts to full-blown visual hallucinations. The most well-known examples include LSD, psilocybin mushrooms, mescaline, and DMT.
LSD
LSD (lysergic acid diethylamide) is a synthetic compound derived from lysergic acid, a substance found naturally in ergot, a fungus that grows on rye grain. It is extraordinarily potent, with active doses measured in micrograms (millionths of a gram), starting around 30 micrograms. Effects begin roughly 30 minutes after swallowing a dose and typically last 6 to 12 hours depending on the amount taken, body weight, and tolerance. LSD is classified as a Schedule I controlled substance in the United States.
Psilocybin Mushrooms
Psilocybin is the primary psychoactive compound in over 200 species of fungi, commonly called “magic mushrooms.” One of the most potent species, Psilocybe semilanceata (the liberty cap), contains roughly 1.8% to 2% psilocybin by dry weight. The body converts psilocybin into psilocin, which then activates the same serotonin receptors as LSD. A typical psychoactive oral dose starts above 3 milligrams of pure psilocybin. Effects generally last 4 to 6 hours, shorter than LSD. Psilocybin is also Schedule I federally, though several U.S. cities and the state of Oregon have created exceptions for supervised therapeutic use. The FDA has granted breakthrough therapy designations to two separate programs studying psilocybin for treatment-resistant depression and major depressive disorder.
Mescaline
Mescaline is a naturally occurring compound found in several cacti, most famously peyote, a small, dome-shaped, spineless cactus native to southern Texas and northern Mexico. It is also found in San Pedro and Peruvian torch cacti, which grow much larger and are widely available as ornamental plants. Dried peyote “buttons” (the harvested tops of the cactus) have been used in spiritual ceremonies by Indigenous peoples of North and South America for thousands of years. Radiocarbon dating of preserved peyote buttons containing psychoactive alkaloids places this practice as far back as 3780 to 3660 BC. In the U.S., the Native American Church holds a legal exemption to use peyote for traditional ceremonial purposes, but mescaline is otherwise a Schedule I substance.
DMT
DMT (dimethyltryptamine) occurs naturally in dozens of plant species and is the primary psychoactive ingredient in ayahuasca, a brew used ceremonially across South America. When injected or inhaled, DMT produces an intense but short experience, often lasting only 15 to 30 minutes. Ayahuasca lasts much longer (4 to 6 hours) because the brew includes a second plant that prevents the body from breaking down DMT in the gut. DMT is listed as a Schedule I substance. Active intravenous doses start at approximately 0.2 mg per kilogram of body weight.
Dissociative Drugs
Dissociatives work through a completely different brain system than classic hallucinogens. Instead of targeting serotonin, they block a type of receptor that normally responds to glutamate, the brain’s primary excitatory chemical messenger. By blocking these receptors, dissociatives disrupt normal communication between brain regions, creating feelings of detachment from the body, the environment, or both. The experience is less about visual patterns and more about feeling disconnected from reality.
Ketamine
Ketamine was developed in the 1960s as a surgical anesthetic and is still used medically today. At lower, sub-anesthetic doses, it produces sensory distortions, feelings of floating, and altered thinking. Research from the early 1990s showed that a single low-dose infusion could produce transient psychotic-like symptoms in healthy adults, including sensory illusions, paranoid thoughts, poor attention, and difficulty finding words. Ketamine is not listed in Schedule I alongside most other hallucinogens; it occupies a unique regulatory position partly because of its recognized medical uses, including a recently approved nasal spray formulation for depression.
PCP
PCP (phencyclidine) was originally developed as an anesthetic but was abandoned for human use because of its intense psychological side effects. It blocks the same glutamate receptors as ketamine but also strongly affects dopamine, the chemical associated with reward and euphoria. This dual action can produce unpredictable behavior, agitation, numbness, and a sense of invulnerability along with hallucinations. PCP effects can last many hours, and high doses carry serious risks including seizures and coma.
DXM
Dextromethorphan (DXM) is the cough suppressant found in many over-the-counter cold medicines. At recommended doses it suppresses coughing, but at doses many times higher than directed it acts as a dissociative hallucinogen through the same glutamate-blocking mechanism as ketamine and PCP. Because it is cheap and widely accessible, DXM misuse is a persistent concern, particularly among adolescents.
Salvia Divinorum
Salvia divinorum is often grouped with dissociatives, but it works through an entirely different mechanism from every other hallucinogen on this list. Its active compound, salvinorin A, activates a specific type of opioid receptor that neither classic hallucinogens nor other dissociatives touch. It does not act on serotonin receptors at all. The experience is typically brief (under 30 minutes when smoked), intensely disorienting, and qualitatively different from other hallucinogens. Users often describe distorted gravity, merging with objects, or entering entirely alternate realities. Salvia’s legal status varies by state; it is not federally scheduled.
Deliriants
Deliriants are the least recreationally popular category of hallucinogens, and for good reason. They work by blocking the brain chemical acetylcholine, which is essential for memory, attention, and distinguishing real perceptions from imagined ones. The hallucinations they produce are qualitatively different from those of classic psychedelics or dissociatives. Rather than geometric patterns or feelings of detachment, deliriants cause fully formed, realistic hallucinations that users often cannot distinguish from reality, along with severe confusion, memory loss, and physical side effects like dangerously elevated heart rate and body temperature.
The most notable deliriant compounds are atropine, scopolamine, and hyoscyamine, all derived from tropane alkaloids found in plants like deadly nightshade (Atropa belladonna), jimsonweed (Datura stramonium), and mandrake. These plants have a long history of both medicinal and poisonous use stretching back thousands of years. Scopolamine is still used medically in small doses for motion sickness patches. Diphenhydramine, the antihistamine in many sleep aids and allergy medications, can also produce deliriant effects at high doses.
How the Three Categories Feel Different
The distinction between these categories matters because the subjective experiences are dramatically different. Classic hallucinogens tend to intensify and warp existing sensory input. Colors become more vivid, surfaces appear to breathe or ripple, and thoughts become deeply interconnected. You remain generally aware that what you’re experiencing is drug-induced.
Dissociatives create a sense of separation. Your body may feel distant or numb, sounds may seem far away, and your sense of self can dissolve. At higher doses, people describe complete out-of-body experiences or entering void-like spaces.
Deliriants are the most disorienting of the three. The hallucinations feel indistinguishable from ordinary reality. People commonly report having full conversations with individuals who aren’t there, or attempting to interact with phantom objects. Unlike classic hallucinogens, deliriants impair your ability to recognize that you’re hallucinating at all.
Risks That Apply Across Categories
All hallucinogens carry the risk of profoundly negative experiences during intoxication, often called “bad trips,” which can involve intense fear, paranoia, or panic. The psychological distress can be severe enough to require emergency care even when the substance itself isn’t physically dangerous.
A condition called hallucinogen persisting perception disorder (HPPD) can develop after use of hallucinogens, particularly classic psychedelics. People with HPPD re-experience perceptual disturbances from their hallucinogen use while completely sober: trailing images behind moving objects, halos around lights, flashes of color, or objects appearing to shift in size. Prevalence is estimated at roughly 4.2% of people who use hallucinogens. A milder form (HPPD I) is brief and not particularly distressing, while a more severe form (HPPD II) can be long-lasting, intrusive, and disabling. Importantly, people with HPPD maintain intact reality testing, meaning they know these perceptions aren’t real even as they experience them.
Physical risks vary sharply between categories. Classic hallucinogens like LSD and psilocybin have very low physical toxicity, and fatal overdoses on these substances alone are essentially unheard of. Dissociatives carry greater physical risks at high doses, including respiratory depression, cardiovascular strain, and loss of consciousness. Deliriants are the most physically dangerous category, with a narrow margin between a dose that produces effects and one that causes organ damage, seizures, or death.