Metabolism of psilocybin and psilocin: clinical and forensic toxicological relevance

Hallucinogens produce dose-dependent alterations in perception, thought and mood without markedly depressing or stimulating psychomotor function; sensory distortions such as synesthesia are common but frank hallucinations are not always present. Psilocybin and its dephosphorylated form psilocin are tryptamine-based indole alkaloids present in many Psilocybe and related mushroom genera; both act primarily at serotonin (5-HT) receptors, notably 5-HT2A, and have low abuse potential compared with other classes of psychoactives. While historically used recreationally, renewed clinical interest has arisen because of preliminary therapeutic signals for anxiety, depression and certain addictions, alongside continuing forensic concerns about acute toxic effects and variable individual responses. Dinis-Oliveira set out to review the metabolic fate of psilocybin and psilocin, with particular attention to major and minor metabolites that are relevant for clinical pharmacology and forensic toxicology. The review aims to collate human and non-human metabolic data, indicate enzymatic pathways and principal urinary metabolites, and discuss implications for detection, pharmacodynamics and potential drug interactions.

An extensive English-language literature search of PubMed was conducted without date limits to retrieve publications on psilocybin, psilocin and their known metabolising enzymes and metabolites. Full-text journal articles and relevant books on "magic mushrooms" and hallucinogens were obtained and cross-referenced to identify additional human and non-human studies. The extracted text does not report additional details such as formal inclusion/exclusion criteria, a PRISMA flow diagram, or a formal risk-of-bias assessment, so the review appears to be a narrative synthesis of available primary studies and reports.

Absorption and distribution data indicate important physicochemical differences between psilocybin and psilocin: the phosphorylated psilocybin is more water soluble, whereas the lipophilic psilocin is absorbed more readily from the gastrointestinal tract and has greater central nervous system bioavailability. Animal studies with 14C-labelled psilocybin suggest incomplete oral absorption (about 50%) and widespread tissue distribution, including brain. Rodent work shows rapid hydrolysis of psilocybin to psilocin in the intestine, implying substantial absorption as psilocin. In humans, psilocin becomes detectable in plasma within 20–40 minutes after oral dosing, with peak concentrations at roughly 80–100 minutes and subjective effects resolving by about 4–6 hours. Reported plasma elimination half-lives are approximately 160 minutes for psilocybin and 50 minutes for psilocin. Excretion and metabolite data from animals and humans indicate that psilocin is largely eliminated as conjugated metabolites. In rats, about 65% of an oral dose of psilocin was recovered in urine and 15–20% in bile and faeces within 8 hours; a fraction (10–20%) persisted longer and metabolites could be detected in urine up to seven days. A controlled human study found 3.4 ± 0.9% of an administered psilocybin dose excreted in urine as free psilocin within 24 hours, while later forensic analyses report that approximately 80% of excreted psilocin appears as psilocin-O-glucuronide, making this conjugate the primary urinary metabolite of forensic relevance. The reviewed evidence therefore supports enzymatic hydrolysis of glucuronide conjugates during laboratory analysis as a means to extend detectability. Metabolic pathways described mirror serotonin metabolism to some extent. Psilocybin is rapidly dephosphorylated by alkaline phosphatase (and other nonspecific esterases) in the gut and possibly blood and kidney to yield psilocin, explaining the view of psilocybin as a prodrug whose in vivo effects are mediated by psilocin. Subsequent oxidative and deaminative steps, catalysed by monoamine oxidase (MAO) and aldehyde dehydrogenase, are proposed to produce 4-hydroxyindole-3-acetaldehyde, 4-hydroxyindole-3-acetic acid and 4-hydroxytryptophol. Minor oxidative pathways yielding bluish o-quinone or iminoquinone products have been reported; these may form enzymatically (e.g. ceruloplasmin, cytochrome oxidases) or non-enzymatically and could be associated with reactive oxygen species, but supporting data are limited. Glucuronidation of the phenolic hydroxyl to psilocin-O-glucuronide is a major detoxification route: intestinal UGT1A10 appears important for first-pass glucuronidation, while UGT1A9 likely predominates once psilocin reaches the circulation; N-glucuronidation was not observed. The review highlights factors that may modify metabolism and subjective effects: MAO inhibitors potentiate psilocin effects and co-consumption of ethanol or tobacco (which lower MAO activity) may prolong or intensify the psychedelic experience. There is also a suggestion that psilocin can competitively inhibit MAO, potentially increasing brain serotonin and lowering 5-HIAA. Analytical and stability issues were emphasised: psilocin is chemically unstable in solution and susceptible to losses in stored blood samples (about 90% decline at room temperature over one week); storage at 4 °C with fluoride improves stability to nearly seven days, whereas freezing whole blood produced unpredictable losses, possibly due to enzyme release during haemolysis. The authors therefore recommend cooling and timely serum separation prior to freezing when analysing psilocin.

Dinis-Oliveira interprets the collated evidence to reaffirm that psilocybin functions primarily as a prodrug and that psilocin is the principal psychoactive species mediating the characteristic serotonergic effects. The predominance of psilocin-O-glucuronide in urine is highlighted as a key finding with both clinical and forensic implications: detection windows can be extended if laboratories hydrolyse glucuronide conjugates, and metabolite profiling can aid qualitative and quantitative interpretation. The author notes considerable interindividual variability in psilocin plasma concentrations after oral psilocybin and emphasises the need for further work to characterise additional metabolites, to clarify the impact of drug–drug interactions and genetic polymorphisms on pharmacokinetics and pharmacodynamics, and to develop more sensitive analytical methods. While the toxicity profile in the literature appears relatively low, Dinis-Oliveira cautions that most safety data are old and do not meet contemporary standards, limiting firm conclusions about risk; controlled modern studies are therefore needed, especially in clinical populations being considered for therapeutic use. The review also points to potential non-serotonergic contributions to effects, such as dopaminergic involvement suggested by imaging and pharmacological interaction studies, and advises that metabolomic approaches may yield insights into mechanisms and help toxicologists interpret complex cases. Finally, the discussion draws attention to additional, less-studied tryptaminic constituents of ‘‘magic mushrooms’’ (baeocystin, norbaeocystin, aeruginascin, bufotenine) whose metabolic fate and pharmacology remain poorly characterised; the author suggests their dephosphorylation to active metabolites is likely but underexplored.

The review concludes that psilocybin is rapidly dephosphorylated to psilocin, which undergoes oxidative deamination and extensive glucuronidation, yielding psilocin-O-glucuronide as the principal urinary metabolite of forensic and clinical importance. Given the large interindividual variability, the limited and dated nature of pharmacokinetic and safety data, and the presence of other bioactive mushroom constituents, the author calls for more controlled pharmacokinetic, metabolomic and safety studies, development of improved analytical methods (including glucuronide hydrolysis where appropriate), and investigation of drug interactions and genetic influences to better inform therapeutic use and toxicological interpretation.