Psilocybin: general

Psilocybin is the major hallucinogenic compound in Psilocybe mushrooms [1][2][3], reported in roughly 200 species [4]. The body converts it to psilocin [1], and psilocybin produces profound alterations in consciousness through the 5-HT2A receptor [5]. Indigenous peoples of Mesoamerica have used psilocybin-containing mushrooms in ritual contexts since long before modern times [6][7]; Western science isolated the compound only in 1958, when Albert Hofmann and colleagues identified it in Psilocybe mexicana [2][4].

Two things matter most before anything else.

First, fatal overdose from pure psychedelics as a class is extremely rare [9][10], and for LSD and psilocybin specifically, no dose has been established above which life is endangered [11]. That is not the same as safe: for psychedelics as a class, taking a very high dose increases the likelihood of a distressing experience [10], and risk increases with high doses, co-ingestion, and underlying health conditions [12]. Overdose deaths from psychedelics as a class typically involve very large doses or mixing with other substances [9].

Second, combination risk is real and underexamined. Psilocybin's activity is thought to be serotonergic [1], and serotonergic psychedelics as a class can cause serotonin toxicity in combination with other serotonergic agents — the serious risk sits with combinations involving an MAOI, while serotonergic drugs that don't contain MAOIs, SSRIs included, are low-risk together [8]. Combining MAOIs with SSRIs specifically can lead to serotonin syndrome [13]. How classic psychedelics interact with antidepressants is still under-researched [13]. Drug interactions increase the risk of harm from psychedelics [8], and interactions with serotonergic medications in particular require careful screening [12].

Beyond the pharmacology, two practical risks deserve attention: what's actually in the material, and where you take it. Substance impurity poses a risk to users of psychedelics [14][10], and adulterants such as methamphetamine and fentanyl have been found in rare cases among submitted psychedelic samples [14]. Drug checking can help identify impurities [14]. Set and setting — who you're with, where you are, whether someone sober is present — are critical determinants of harm from psychedelics [11][10].

References

1. psilocybin | Ligand page | IUPHAR/BPS Guide to PHARMACOLOGY — IUPHAR/BPS
2. Psilocybin - American Chemical Society — American Chemical Society
3. Psilocybin — Encyclopaedia Britannica
4. Psilocybin | Johns Hopkins Psychiatry Guide — Johns Hopkins Medicine
5. Psychedelic Therapy: A Primer for Primary Care Clinicians-Psilocybin - PubMed — PubMed (journal article)
6. Psilocybin and hallucinogenic mushrooms | CNS Spectrums — Cambridge University Press
7. Psilocybin for Mental Health and Addiction: What You Need To Know | NCCIH — National Center for Complementary and Integrative Health (NIH)
8. Serotonin toxicity of serotonergic psychedelics — PubMed
9. Adverse effects of psychedelics: From anecdotes and misinformation to systematic science — Journal of Psychopharmacology
10. Understanding Risk in the use of Psychedelics — UC Berkeley Center for the Science of Psychedelics
11. Hofmann vs. Paracelsus: Do Psychedelics Defy the Basics of Toxicology?—A Systematic Review — PMC / National Library of Medicine
12. Pharmacological Properties of Psychedelics with a Special Focus on Potential Harms — PubMed
13. Drug–drug interactions involving classic psychedelics: A systematic review — Journal of Psychopharmacology
14. Harm reduction practises for users of psychedelic drugs: a scoping review — Harm Reduction Journal