A hallucinogen is any substance that significantly alters perception, mood, and thought, often causing a person to see, hear, or feel things that aren’t there. These drugs range from compounds found naturally in mushrooms and cacti to synthetic chemicals made in laboratories. They fall into two broad categories: classic hallucinogens like LSD and psilocybin, and dissociative drugs like ketamine and PCP.
Classic Hallucinogens vs. Dissociative Drugs
Classic hallucinogens and dissociative drugs both distort reality, but they do it through completely different brain pathways and produce noticeably different experiences.
Classic hallucinogens work primarily by activating serotonin receptors in the brain, specifically a receptor subtype involved in mood, perception, and cognition. LSD, psilocybin (the active compound in “magic mushrooms”), mescaline (from peyote and San Pedro cacti), and DMT (found in ayahuasca) all belong to this group. Their hallmark effects include vivid visual distortions, intensified colors, geometric patterns, and deep shifts in emotional and abstract thinking.
Dissociative drugs take a different route. They block a type of receptor involved in glutamate signaling, the brain’s primary excitatory chemical messenger. Ketamine and PCP are the most well-known examples. By interfering with glutamate, these substances create a sense of detachment from the body and environment. People often describe feeling disconnected from themselves, as if watching their own experience from the outside. DXM, found in some over-the-counter cough medicines, also falls into this category. One outlier is salvia divinorum, which is classified as a dissociative but works by activating a specific type of opioid receptor rather than blocking glutamate.
Natural and Synthetic Sources
Hallucinogens come from a surprisingly wide range of natural sources. Psilocybin is produced by over 200 species of fungi, with the genus Psilocybe being the best known. Other mushroom genera, including Panaeolus, also contain psilocybin and its psychoactive relative psilocin. Mescaline occurs naturally in the peyote cactus and several related species. DMT is found in numerous plants used in South American ayahuasca brews, and morning glory seeds contain a related compound called LSA.
On the synthetic side, LSD was first created in a laboratory in 1938 and remains one of the most potent hallucinogens by weight. PCP and ketamine were originally developed as anesthetics before their dissociative properties became widely recognized. Newer synthetic compounds, sometimes sold as “research chemicals,” continue to appear, many of them designed to mimic the effects of classic hallucinogens.
What the Experience Feels Like
People under the influence of hallucinogens commonly report rapid, intense emotional swings alongside dramatic changes in how they perceive the world. Visual effects can include shifting patterns, intensified colors, trails behind moving objects, and surfaces that appear to breathe or ripple. Some people experience synesthesia, where senses blend together: music might produce visual patterns, or textures might seem to have a taste.
The sense of time often warps. Minutes can feel like hours. Thoughts may become unusually abstract or feel profoundly meaningful in ways that are hard to articulate afterward.
One of the most striking and well-studied effects is ego dissolution, a weakening or complete loss of the feeling of being a distinct self separate from the world. People describe the boundaries between themselves and their surroundings becoming blurry, or the sense of “I” disappearing entirely. This can range from a mild softening of self-awareness to a full experience of merging with everything around them, sometimes described as “cosmic consciousness.” Ego dissolution is not all-or-nothing. Different aspects of self-awareness, such as the feeling of owning your thoughts or sensing where your body ends, can be disrupted independently and to different degrees.
Duration and Intensity by Substance
How long a hallucinogenic experience lasts varies dramatically depending on the substance. Psilocybin typically produces effects lasting 4 to 6 hours. LSD lasts considerably longer, usually 9 to 12 hours. Mescaline is similar to LSD in duration, around 12 hours. DMT, when smoked, is the shortest-acting classic hallucinogen, with intense effects lasting only 15 to 30 minutes, though ayahuasca (which delivers DMT orally with a chemical that slows its breakdown) extends the experience to several hours.
A clinical crossover study comparing these substances found that 100 micrograms of LSD, 20 milligrams of psilocybin, and 500 milligrams of mescaline produced roughly equivalent subjective effects at those doses. Lower doses of mescaline (300 mg) produced noticeably weaker peak effects than LSD or psilocybin at their standard doses. This gives a sense of relative potency: LSD is active in millionths of a gram, while mescaline requires hundreds of milligrams to produce comparable effects.
How Classic Hallucinogens Work in the Brain
Classic hallucinogens produce their perception-altering effects primarily by binding to serotonin receptors called 5-HT2A receptors, which are concentrated in brain regions involved in sensory processing and higher-order thinking. When a hallucinogen locks onto these receptors, it triggers a cascade of chemical signals inside the neuron, including the release of stored calcium and the activation of specific signaling proteins.
What makes hallucinogens different from other chemicals that also activate the same receptor is a concept called biased signaling. Hallucinogenic compounds activate a particular secondary signaling pathway that non-hallucinogenic compounds do not, even when both are stimulating the same receptor on the same neurons. This selective activation pattern appears to be what separates a hallucinogenic experience from a non-hallucinogenic one, and it explains why not every drug that interacts with serotonin receptors produces hallucinations.
Risks and Adverse Effects
The most commonly discussed acute risk is a “bad trip,” an experience dominated by intense fear, paranoia, confusion, or disturbing imagery. Bad trips are more likely with higher doses, unfamiliar settings, or pre-existing anxiety.
A less common but more persistent risk is hallucinogen persisting perception disorder, or HPPD. This condition involves visual disturbances that continue long after the drug has left the body: trailing images, halos around objects, flickering in peripheral vision, or visual “snow.” HPPD is estimated to affect roughly 4 to 4.5 percent of people with a history of hallucinogen use. To meet the clinical definition, the visual disturbances must cause significant distress and interfere with daily functioning, and they can’t be explained by another medical condition. For most people, these effects are mild and fade over time, but in some cases they persist for months or years.
Dissociative drugs carry additional risks. At high doses, PCP can cause severe agitation, psychosis, and dangerous increases in body temperature and heart rate. Ketamine, with repeated use, can cause serious bladder damage and cognitive problems. Both PCP and ketamine carry a risk of dependence that classic hallucinogens generally do not.
Legal Status
In the United States, most classic hallucinogens are classified as Schedule I controlled substances, meaning they are considered to have high abuse potential and no accepted medical use under federal law. LSD, psilocybin, mescaline (with a religious exemption for certain Native American Church ceremonies), peyote, and MDMA (ecstasy) all fall into this category. Ketamine is an exception: it’s a Schedule III substance and is legally used as an anesthetic and, increasingly, for treatment-resistant depression.
A handful of U.S. cities and states have moved to decriminalize psilocybin or reduce enforcement priorities around naturally occurring psychedelics, though federal prohibition remains in place.
Therapeutic Research
Hallucinogens are the subject of a growing wave of clinical research. The FDA has granted “breakthrough therapy” designation to two formulations of psilocybin being studied for depression, a status that accelerates the review process for drugs that may offer substantial improvement over existing treatments. Researchers are also studying psilocybin as a potential treatment for alcohol use disorder and for helping people quit smoking. Ketamine is already being used off-label to treat patients with severe depressive episodes who haven’t responded to standard antidepressants.
These studies represent a significant shift from decades of near-total restriction on hallucinogen research, though none of these substances (aside from ketamine) have received full FDA approval as medicines yet.