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What is psilocybin? Mushrooms, research and risks

BySafe Ceremonies

Psilocybin is the main psychedelic compound in the mushrooms people call “magic mushrooms” or “shrooms.” It is offered in mushroom ceremonies and retreats, in the legal programs some US states have set up, and in clinical trials testing it as a treatment for depression and other conditions. Those are very different settings, and the same compound can be a very different experience in each.

This guide explains what psilocybin is, where its use comes from, what happens in the body, what the research has shown so far and the risks that are well documented. It doesn’t cover doses, how to find or prepare mushrooms, or whether you should take it.

What it is

Psilocybin occurs naturally in a number of mushroom species. Chemically it belongs to the tryptamine family, the same family as DMT. Psilocybin itself is mostly a “pro-drug”: once swallowed, the body removes part of the molecule and turns it into psilocin, which is the compound that acts on the brain.

Psilocin works mainly by switching on one type of serotonin receptor, the 5-HT2A receptor. A review in Pharmacological Reviews describes a human study in which a drug that blocks this receptor, ketanserin, stopped psilocybin’s effects. That is strong evidence the 5-HT2A receptor is where the main action starts. Our guide on how psychedelics work in the brain goes further into this.

Mushrooms vary in how much psilocybin they contain, by species, by growing conditions and from one batch to the next. Dried mushrooms bought informally carry no reliable label, so the strength of what you’re given is often a guess.

Where its use comes from

The US National Center for Complementary and Integrative Health (NCCIH) notes that psilocybin mushrooms have been used traditionally in Mexico and Central America for thousands of years.

In the late 1950s the Swiss chemist Albert Hofmann made psilocybin in the lab, and it was supplied to researchers under the name Indocybin. After psilocybin was placed in Schedule I of the US Controlled Substances Act in 1970, research declined sharply. Clinical studies have since returned, and the NCCIH page linked below summarizes where they stand.

What it does and how long it lasts

In a 2023 Swiss study that gave healthy volunteers psilocybin, LSD and mescaline on separate days, psilocybin’s effects lasted about five hours on average, the shortest of the three. Mushrooms eaten whole or as a tea can take effect at a slightly different pace, and ceremonies usually run longer than the effects themselves.

The US National Institute on Drug Abuse (NIDA) describes psychedelics as bringing on vivid visions and changes in the sense of self, with emotions that range from intense happiness and connection to fear, anxiety and confusion. Physical effects can include nausea, headache and changes in heart rate. Safety guidelines written by Johns Hopkins researchers add that psychedelics can moderately raise pulse and blood pressure.

What the research has shown so far

The research is promising in some areas and limited in others.

  • The FDA gave psilocybin-assisted therapy “breakthrough therapy” status for treatment-resistant depression in 2018 and for major depressive disorder in 2019. That status speeds up development. It is not an approval.
  • In a 2022 trial of 233 people with treatment-resistant depression, published in the New England Journal of Medicine, a single high dose given with psychological support lowered depression scores more than a very low dose over three weeks. The effect was not clearly maintained at 12 weeks. 77% of participants had side effects such as headache, nausea and dizziness, and suicidal thoughts, suicidal behavior or self-injury occurred in every group. The authors called for larger, longer trials.
  • NCCIH summarizes early evidence for depression, anxiety and distress in people with life-threatening illness, and alcohol use disorder, always alongside psychotherapy.
  • Blinding is a known weak spot. Most people can tell whether they got a psychedelic, and a 2021 review argued that this, together with high expectations, probably makes the effects in published trials look larger than they are.

When this guide was checked in October 2026, psilocybin had not been approved by the FDA as a medicine, though late-stage trials were well under way. Even when a psilocybin medicine is approved, it will be a specific product given under medical supervision. That says little about mushrooms served at a retreat.

Known risks

Clinical trials screen people carefully and keep trained staff with them throughout, which keeps serious problems rare. Outside the lab, the picture is rougher. In a survey of 1,993 people about their single most difficult psilocybin mushroom experience:

  • 39% ranked it among the five most challenging experiences of their life.
  • 11% said they had put themselves or others at risk of physical harm during it, and 2.6% had been physically aggressive.
  • 2.7% got help at a hospital or emergency department.
  • Of those whose experience was more than a year earlier, 7.6% had sought treatment for psychological symptoms they linked to it.

The same survey found that most respondents also said they had benefited from the experience. Both things can be true, which is why preparation and support matter so much.

Other risks named by NCCIH and the Johns Hopkins guidelines:

  • Anxiety, paranoia and, rarely, lasting psychosis. NCCIH says psilocybin is not safe for people with psychotic conditions such as schizophrenia, and research studies exclude people with a personal or family history of psychosis or bipolar disorder.
  • Raised blood pressure and heart rate, which matters if you have heart disease.
  • Fatal poisoning from mistaking a toxic mushroom for a psilocybin one.
  • Products that aren’t what they claim. NCCIH warns that illicit products may be adulterated, including with fentanyl.
  • Interactions with other drugs and medicines. A 2024 systematic review found that psychedelics combined with other drugs can have weaker, stronger or unchanged effects. Talk to the person who prescribes your medicines, and don’t stop any of them on a center’s advice.

For several hours you may also be less able to look after yourself. A ceremony with too few sober staff, open water nearby or no way to call for help adds risks of its own. Our red flags guide covers what to watch for.

A note on the law

Psilocybin is a Schedule I controlled substance under US federal law. Some US cities and states have decriminalized or deprioritized possession, and Oregon and Colorado have set up regulated programs for supervised use. Other countries take very different approaches. Laws change, and this is a description, not legal advice. Our guide on psilocybin in Oregon and Colorado covers the state programs.

Questions worth asking a center

  • What exactly will I be given, and how do you know its strength?
  • How do you screen for mental health history, heart problems and medicines?
  • How many trained, sober staff are present for how many participants?
  • What happens if someone panics, tries to leave or becomes unwell?
  • What support do you offer in the days and weeks after?

More guides

Reviews from past participants show how a group handles screening, hard moments and the days after, which matters more than the setting it advertises. You can search the Safe Ceremonies listings, including psilocybin and mushroom ceremonies and psychedelic-assisted therapy.

Sources

Photo: Mossy Log on Forest Floor by Channel Raven, CC0 1.0.

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