Psilocybin

• 1. Introduction

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  Psilocybin is a compound found in more than 200 species of mushrooms, and its safety record is oddly lopsided. Only two deaths from overdosing on hallucinogenic mushrooms alone have ever been reported in the scientific literature. Even those two may involve factors other than psilocybin. Yet for decades the substance was treated as a danger to be locked away, and possession of the mushrooms is still outlawed in most countries. How did a fungus used in ceremonies in Mexico end up in a Swiss laboratory? Why does it bend a person's sense of time, and why did Harvard's experiments with it end in scandal? And why are regulators now weighing whether it should become a prescribed medicine?

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• 2. Old Rumors, New Chemistry

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  In Algeria, the Tassili Mushroom Figure has been argued to show an early mushroom cult. Pictographs about 6,000 years old near Villar del Humo, Spain, show mushrooms that some link to Psilocybe hispanica. Historians urge caution. Claims of widespread ancient use are largely exaggerated, shaped by modern idealization and ideology. The reliable evidence points to ritual use in pre-Columbian Mexico, and little anywhere else.

  The Dominican friar Diego Durán wrote around 1581 that mushrooms were eaten at celebrations marking the accession of the Aztec emperor Moctezuma II in 1502. The Franciscan friar Bernardino de Sahagún described merchants who celebrated a successful trip by eating mushrooms to bring on visions. After the Aztecs fell, the Spanish banned such rites as pagan idolatry. For the next four centuries, the Indians of Mesoamerica hid their use from the authorities.

  Europe left only scraps. The botanist Carolus Clusius described a bolond gomba, or crazy mushroom, that rural Hungarians used in love potions. The English botanist John Parkinson wrote of a foolish mushroom in his 1640 herbal. The first reliably documented poisoning with Psilocybe semilanceata came in 1799, when a British family cooked a meal from mushrooms picked in London's Green Park.

  The modern chapter opened with R. Gordon Wasson, an American banker and amateur ethnomycologist, and his wife Valentina, a physician. They studied mushroom rituals among the Mazatec people of Huautla de Jiménez, Mexico. In 1957, Wasson described his visions in a Life magazine article titled "Seeking the Magic Mushroom." The French mycologist Roger Heim joined a follow-up expedition that year, identified several of the mushrooms as Psilocybe species, and grew them in France.

  Heim sent samples to Albert Hofmann, a chemist at the Swiss firm Sandoz who had made LSD in 1938. Hofmann's group isolated psilocybin and psilocin from Psilocybe mexicana and published the result in 1958. Hofmann helped confirm the active compounds by swallowing mushroom extracts himself. His team then built related molecules. Two of them, ethocybin and CZ-74, wore off in about three and a half hours, roughly half as long as psilocybin, which suited psycholytic therapy. Sandoz also sold pure psilocybin as Indocybin to clinicians and researchers worldwide, with no reports of serious complications.

  Hofmann never argued for legalizing these drugs or using them medically. In a 1979 book he called LSD his problem child.

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• 3. Harvard and the Backlash

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  At Harvard University in the early 1960s, Timothy Leary and his associates Ralph Metzner and Richard Alpert turned the campus into a testing ground. Alpert later changed his name to Ram Dass. Leary obtained synthesized psilocybin from Hofmann through Sandoz. The Concord Prison Experiment and similar work suggested promising results for psychiatry.

  Leary's group also studied why the same dose could feel so different from person to person. They gave psilocybin to 175 volunteers in a room meant to resemble a comfortable living room. Of these, 98 filled out questionnaires about their experience. People with earlier experience of the drug reported more pleasant sessions. Groups of more than eight felt less supportive, while groups of fewer than six reported better reactions. Leary proposed that psilocybin heightens suggestibility. A later review by Jos ten Berge, published in 1999, affirmed that dose, mindset and surroundings drive the outcome.

  The politics soon overtook the science. A 2008 review of safety guidelines judged that Leary's and Alpert's firing from Harvard, and their later advocacy of hallucinogens, undermined objective study. The press covered these drugs unfavorably as unauthorized use rose. In 1966, US laws banned producing, trading or ingesting hallucinogens, and Sandoz stopped making LSD and psilocybin that year. A 1970 federal act made LSD, peyote, psilocybin and other hallucinogens illegal for any purpose, research included. The 1971 United Nations Convention on Psychotropic Substances placed psilocybin in Schedule I.

  Funding dried up, and scientists who worked with psychedelics risked being professionally marginalized. Meanwhile, the mushrooms were about to find a route into ordinary kitchens.

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• 4. Mystical Experiences

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  On a day in 1962 at Harvard Divinity School, Walter Pahnke ran what became the Marsh Chapel Experiment under Leary's supervision. Almost all the divinity students who received psilocybin reported profound religious experiences. One volunteer was Huston Smith, author of several comparative religion textbooks, who called his experience "the most powerful cosmic homecoming I have ever experienced."

  A 25-year follow-up found that every subject given psilocybin still described a genuine mystical element. Many called it one of the high points of their spiritual lives. Rick Doblin judged the study partly flawed, because the double-blind procedure was carried out incorrectly and some questionnaire items were imprecise. Even so, he said it cast serious doubt on the idea that drug-triggered mystical experiences are inferior to ones reached without drugs. The psychiatrist William A. Richards echoed that view in a 2007 review. Pahnke had categorized the common features of such states, hoping to describe a universal experience free of cultural interpretation, and some scholars see little difference from meditation or holotropic breathwork.

  Roland Griffiths and colleagues at the Johns Hopkins School of Medicine set out to test this more rigorously. Asked how his work compared with Leary's, Griffiths answered: "We are conducting rigorous, systematic research with psilocybin under carefully monitored conditions, a route which Dr. Leary abandoned in the early 1960s." Their study, funded by the National Institute of Drug Abuse and published in 2006, drew praise for its design.

  Thirty-six volunteers with no hallucinogen experience took psilocybin in one session and methylphenidate, sold as Ritalin, in another. Using a questionnaire developed by Ralph W. Hood, 61% reported a complete mystical experience after psilocybin, against 13% after methylphenidate. Two months later, 79% reported moderately to greatly increased life satisfaction. Yet about 36% had strong to extreme fear at some point, and for 13% of the total it dominated the whole session. The researchers managed this easily, and it left no lasting harm.

  A follow-up 14 months later found that almost a third called it the most meaningful or spiritually significant event of their lives. More than two-thirds placed it among their top five. Those with mystical experiences scored about 4 percentage points higher on the trait of Openness, a trait that normally holds steady in adults. A separate 2010 web questionnaire reached 503 users, and 60% of them reported a lasting positive effect on their well-being.

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• 5. Mind, Time and Brain

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  Users often say that "minutes appear to be hours" or that "time is standing still." Laboratory tests back this up. Psilocybin significantly impairs the ability to judge intervals longer than 2.5 seconds, and to keep pace with beats more than 2 seconds apart. It also lowers a person's preferred tapping rate. This fits the role of the prefrontal cortex in timing, though the underlying neurochemistry is not known with certainty.

  With eyes closed, a person on a low dose may see multicolored geometric shapes and vivid imagined scenes. Some report synesthesia, such as feeling touch while looking at colors. At higher doses, open-eye visions can be detailed but are rarely mistaken for reality. The author Peter Stafford, writing in the Psychedelics Encyclopedia, compared it with LSD and wrote that "The psilocybin experience seems to be warmer, not as forceful and less isolating."

  The chemistry starts with a trick. Psilocybin itself is biologically inactive. Enzymes called phosphatases strip off its phosphate group in the stomach, gut and tissues, and what remains is psilocin. That makes psilocybin a prodrug. Because of its phosphate group, psilocybin cannot easily cross the blood-brain barrier, but psilocin can. Psilocin resembles serotonin and activates serotonin receptors, and one of them, the 5-HT2A receptor, is responsible for the hallucinations. Drugs such as ketanserin and risperidone block that receptor and can shut the effects down.

  Brain imaging adds a second layer. In studies of single doses, regions of the default mode network, tied to introspection and sense of self, communicate less with one another. That network is linked to rumination and a poor self-image in major depressive disorder. Regions in the theory of mind network, which handle social inference, become more active. Trials of mescaline and LSD showed no significant difference. In animals, psilocin also promotes the growth of new neural connections after a single dose.

  Taken by mouth, effects begin in about 20 to 50 minutes, peak at one to two hours and last four to six hours. A close chemical cousin shows why the details matter. Bufotenin, found in the skin of the Colorado River toad, lacks the internal pseudo-ring that helps psilocin slip into the brain.

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• 6. Risks and Safety

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  In one study, 31% of volunteers given a high dose reported significant fear, and 17% had passing paranoia. At Johns Hopkins, one-third of those given a high dose felt anxiety or paranoia, while negative experiences were rare at moderate doses. A bad trip is mainly a reaction of fear. Taking the drug in a low mood, in an unsupportive place, or together with alcohol makes it more likely.

  A 2005 survey of people in the United Kingdom found that almost a quarter of those who had used mushrooms in the past year had a panic attack. Over a quarter had nausea or vomiting, which the source attributes to the mushroom rather than to psilocybin. Physical effects are usually mild. Peak heart rate rises by about 5 beats per minute and peak systolic blood pressure by 10 to 15 mm Hg. Pupil dilation appeared in 93% of subjects in early studies.

  People with a history of psychosis, schizophrenia, bipolar disorder or borderline personality disorder face the greatest psychiatric risk. Pregnant or breastfeeding women are advised to avoid it because research is thin. Some medicines change the experience. Antipsychotics and certain antidepressants can block the high, which is why they are nicknamed "trip killers." Buspirone markedly reduces it. Monoamine oxidase inhibitors can intensify it and raise the risk of toxicity. A trial pairing psilocybin with midazolam found that the sedative clouded the effects and impaired memory of the session.

  Tolerance builds and fades fast. Taking psilocybin more than about once a week blunts its effects, and spacing doses several days apart avoids that. Repeated use does not cause physical dependence. A 2010 Dutch study ranked psilocybin mushrooms the least harmful of 20 drugs, a list that included alcohol, heroin and tobacco. One long-term worry is cardiac fibrosis from frequent use, which applies more to chronic microdosing than to doses spaced months apart.

  In rats, the median lethal dose is 280 mg/kg, about 1.5 times that of caffeine. A 60 kg person would need nearly 1.7 kg of dried Psilocybe cubensis to reach that level. The estimated human lethal dose is 2,000 to 6,000 mg, around 200 times a typical recreational dose of 10 to 30 mg. The Registry of Toxic Effects of Chemical Substances gives psilocybin a therapeutic index of 641, compared with 199 for aspirin and 21 for nicotine. Most of the few fatal incidents tied to mushroom use involve other drugs, especially alcohol.

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• 7. The Mushrooms Themselves

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  In a 2000 review, Gastón Guzmán and colleagues sorted the psilocybin mushrooms into 11 genera. Psilocybe led with 116 species, followed by Gymnopilus with 14 and Panaeolus with 13. By 2005 Guzmán had raised his count of psilocybin-containing Psilocybe to 144 species. Mexico holds 53 of them, the United States and Canada 22, Australia and nearby islands 19, Europe 16, Asia 15 and Africa 4. Part of this diversity is thought to come from the psilocybin gene cluster hopping between unrelated mushroom species.

  Psilocybe semilanceata is the most widely distributed of all, found across Europe, North America, Asia, South America, Australia and New Zealand. It is entirely absent from Mexico. Strength varies enormously. P. azurescens tops the list with a reported maximum of 1.78% of dry weight, while P. cubensis reaches 0.63% and P. liniformans only 0.16%. Overall potency ranges from almost nothing to 2.5%. Caps carry more than stems, and the spores carry none. Young, small mushrooms are stronger than mature ones because most of the compound is made early in their growth.

  Storage matters just as much. Dried mushrooms keep their strength for months or years, while fresh ones kept for four weeks retain only traces. In one study, dried P. semilanceata specimens aged 11, 33 and 118 years held 0.84%, 0.67% and 0.014% psilocybin. Many species bruise blue when handled, but this is not a reliable test of identity or strength.

  Researchers once thought mushrooms built the molecule in four steps, ending with phosphorylation of psilocin. Newer work in P. cubensis shows that phosphorylation comes third and that neither dimethyltryptamine nor psilocin is an intermediate. Four enzymes, PsiD, PsiH, PsiK and PsiM, carry out the sequence. Scientists have since engineered Escherichia coli and yeast to make psilocybin.

  Laboratory synthesis has scaled up as well. Shirota and colleagues reported a gram-scale method in 2003. In 2020, Sherwood and colleagues improved on it to reach the kilogram scale with cheaper reagents.

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• 8. Return to Medicine

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  In 1976, a book called Psilocybin: Magic Mushroom Grower's Guide appeared under the pseudonyms O. T. Oss and O. N. Oeric. Its real authors were Jeremy Bigwood, Dennis J. McKenna, K. Harrison McKenna and Terence McKenna. It adapted a technique for growing the common edible mushroom Agaricus bisporus to Psilocybe cubensis, using ordinary kitchen tools. More than 100,000 copies were sold by 1981. For the first time, a layperson could grow a potent mushroom at home, and psilocybin became the entheogen of choice in the 1970s despite the laws.

  In the late 1990s and early 2000s, legal uncertainty let European retailers sell psilocybin sclerotia, called truffles, in smartshops in the Netherlands, the UK and online. Since 2001, six EU countries have tightened their laws in response. Research also revived in the 1990s, especially in Europe, helped by brain imaging and the use of psilocybin to study how hallucinations and psychotic symptoms arise.

  The medical case is now the main story. In 2023, Australia's Therapeutic Goods Administration approved psilocybin for treatment-resistant depression. In the United States it is in late-stage trials for major depressive disorder and treatment-resistant depression. In April 2026, the FDA issued a National Priority Voucher for both conditions. A decision is expected by the end of 2026, possibly as early as summer 2026.

  The evidence is promising but uneven. At higher doses and with psychological support, single doses may produce rapid, long-lasting antidepressant effects that beat placebo. Against SSRIs such as escitalopram, the advantage may be modest or absent, and trials often have small samples and bias. The drug is also being studied for substance use disorders, obsessive-compulsive disorder and cluster headaches.

  Doses in this field are measured carefully. Psychedelic doses run from 5 to 40 mg by mouth. About 20 mg counts as a good effect dose, and 30 to 40 mg can dissolve the sense of self. A microdose is below 2.5 mg. Use is far smaller than for cannabis. A June 2024 RAND Corporation report put psychedelic use days two orders of magnitude below cannabis. The coming approval decision will show whether a compound pushed out of research in 1970 can return as a prescribed medicine.

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