Hallucinogens are a broad class of drugs that alter perception, mood, and thought processes. They can cause you to see, hear, or feel things that aren’t there, or dramatically distort your experience of things that are. The term is loosely applied to a range of substances that work in different ways in the brain, from psilocybin (the active compound in “magic mushrooms”) to ketamine to LSD.
The Two Main Categories
Researchers generally split hallucinogens into two primary groups based on how they affect the brain, plus a handful of substances that don’t fit neatly into either one.
Psychedelics (also called “classic psychedelics”) primarily change how the brain processes serotonin, a chemical messenger involved in mood, perception, and sense of self. This group includes psilocybin, LSD, DMT, and mescaline. At sufficient doses, these substances can produce vivid visual experiences, vibrant colors and geometric patterns, relived memories, and a shifted sense of identity. People often describe feelings of deep connection or insight.
Dissociatives work on a different brain system entirely. Drugs like ketamine and PCP block receptors involved in transporting glutamate, another chemical messenger. Rather than producing vivid visions, dissociatives tend to make people feel detached from their own body and surroundings. Users commonly describe distorted vision and hearing, a sensation of floating, or feeling like they’re watching themselves from the outside.
A few substances blur the lines. MDMA (commonly called ecstasy), ibogaine, and salvia each affect multiple brain systems and can produce psychedelic effects, dissociative effects, or both.
How Psychedelics Work in the Brain
Classic psychedelics come from several different chemical families (tryptamines, phenethylamines, lysergamides) but all converge on the same target: a specific serotonin receptor called 5-HT2A. This receptor sits on the surface of brain cells and normally responds to serotonin. Psychedelics activate it in an unusually potent way, triggering a cascade of signaling inside the cell that reshapes how the brain processes sensory information and constructs your sense of reality.
The evidence that this single receptor is the key to the psychedelic experience is strong. When researchers give people a drug that blocks the 5-HT2A receptor before administering psilocybin or LSD, the psychedelic effects are largely eliminated. The intensity of hallucinations, the altered sense of self, the visual distortions: all of it depends on that receptor being activated.
Dissociatives take a completely different route. Ketamine, for example, is a noncompetitive blocker of NMDA receptors. It essentially plugs the channel these receptors use, reducing glutamate signaling. One leading theory is that ketamine preferentially quiets inhibitory brain cells first, which paradoxically unleashes a surge of activity in excitatory brain cells. This disrupted balance between excitation and inhibition likely explains the strange feeling of being “unplugged” from your body and environment.
What Hallucinogens Feel Like
The subjective experience varies enormously depending on the substance, the dose, and the person. With classic psychedelics like LSD and psilocybin, the most commonly reported effects are vivid visual changes: colors appear more intense, objects may seem to breathe or shift, and geometric patterns can overlay your vision. Many people describe reliving memories with unusual clarity or experiencing emotions with heightened intensity. Your sense of time may stretch or compress, and the boundary between yourself and your surroundings can feel thinner or even disappear.
Dissociative experiences feel quite different. Instead of an explosion of sensory detail, you may feel a numbing detachment. Vision and hearing become distorted rather than enhanced. At higher doses, people describe complete disconnection from physical sensation, sometimes called a “hole” experience with ketamine.
On the physical side, most hallucinogens produce a temporary increase in heart rate and blood pressure. Clinical trials have measured these effects precisely. Psilocybin typically raises heart rate by about 10 beats per minute above baseline and can push systolic blood pressure into the 140s. LSD shows similar dose-dependent increases. DMT, especially when given intravenously, can cause more dramatic spikes: peak blood pressures of 159/98 mmHg and heart rates near 120 beats per minute have been recorded, though these surges are brief. In all cases, these cardiovascular effects are transient and generally resolve as the drug wears off.
Toxicity and Overdose Risk
Classic psychedelics have an unusually wide safety margin compared to most other recreational drugs. When researchers ranked commonly abused substances by the ratio between a typical dose and a lethal dose, several hallucinogens had the highest safety ratios of any category, while intravenous heroin had the lowest. The difference between the safest and most dangerous substances spanned more than a factor of 100.
That said, “low physiological toxicity” does not mean “safe.” The psychological risks are real. A difficult psychedelic experience (a “bad trip”) can involve intense fear, paranoia, or confusion. People in this state can make dangerous decisions, and the experience itself can be deeply distressing. The cardiovascular effects, while typically mild in healthy people, could pose risks for someone with an underlying heart condition.
Hallucinogen Persisting Perception Disorder
A small percentage of hallucinogen users develop lingering visual disturbances long after the drug has left their system. This condition, called HPPD, involves symptoms like trailing images behind moving objects, halos around lights, flashes of color, objects appearing larger or smaller than they are, and false perceptions of movement at the edges of your vision. These aren’t flashbacks in the dramatic sense often portrayed in media. They’re subtle, persistent visual glitches that can range from mildly annoying to genuinely distressing.
Reliable prevalence numbers are hard to pin down. The DSM-5 (the standard psychiatric diagnostic manual) suggests that about 4.2% of hallucinogen users experience HPPD-like symptoms. Among people who use multiple drugs, that estimate climbs much higher, with some research suggesting up to 50% of polydrug users may experience some version of these symptoms. HPPD is diagnosed only after ruling out neurological conditions like epilepsy and other psychiatric disorders.
Therapeutic Research
Several hallucinogens are being studied as potential treatments for mental health conditions. Psilocybin has received breakthrough therapy designation from the FDA for both treatment-resistant depression (in 2018) and major depressive disorder (in 2019). That designation speeds up the review process but is not an approval. As of the most recent analysis, 134 clinical trials involving psilocybin have been registered, covering more than 50 potential uses, but zero have resulted in FDA marketing approval. The most advanced trials (Phase 2/3 and Phase 3) are still several years from potential approval, assuming positive outcomes.
Ketamine occupies an interesting middle ground. It is FDA-approved as an anesthetic, and a nasal spray form of a closely related molecule has been approved for treatment-resistant depression. However, ketamine itself is not approved for depression or chronic pain, despite growing off-label use for both.
Legal Status
Most classic hallucinogens are Schedule I controlled substances under federal law, the most restrictive category. This includes LSD, psilocybin, DMT, and MDMA. Schedule I classification means the government considers them to have a high potential for abuse, no currently accepted medical use, and a lack of accepted safety for use even under medical supervision. Ketamine is classified less restrictively as Schedule III, reflecting its established medical use as an anesthetic. Some states and cities have moved to decriminalize possession of certain psychedelics, particularly psilocybin, but federal scheduling remains unchanged.