Psilocybin - Summary of knowledge and new perspectives

Psilocybin and its active metabolite psilocin, the principal psychoactive constituents of hallucinogenic mushrooms, have been investigated both as experimental tools for modelling psychosis and as candidate therapeutic agents. Historical use stretches back millennia in ritual contexts, and the compounds were introduced to Western science in the mid-20th century; however, widespread recreational use led to restrictions and a long hiatus in human research. Recent decades have seen a revival of experimental work because psilocybin combines relatively well characterised pharmacology with oral bioavailability and a tolerable safety profile in controlled settings. Tylš and colleagues set out to assemble a comprehensive, up-to-date synthesis of knowledge about psilocybin. The paper reviews chemical structure, metabolism and pharmacokinetics, receptor pharmacodynamics, animal and human behavioural effects, neurophysiological and neuroimaging findings, toxicity and safety issues, and emerging clinical applications. The stated aim is to bring together past and contemporary findings to clarify both the research utility and therapeutic potential of psilocybin.

The extracted text does not present a dedicated Methods section detailing search strategy, inclusion criteria or systematic review procedures. From the content it is apparent that the paper is a narrative review that integrates historical reports, preclinical experiments, human experimental studies, electrophysiology and neuroimaging work, and clinical case reports and pilot trials. Material summarised includes chemical and pharmacokinetic data, receptor-binding and functional pharmacology, behavioural pharmacology in animals, controlled human psychopharmacology experiments (including psychometric scales), electrophysiological (EEG/MEG) and neuroimaging (PET/fMRI) investigations, toxicological findings, and small clinical and case-series reports of therapeutic applications. Where relevant, numerical values (for doses, timing, plasma levels, toxicology indices and prevalence estimates of adverse psychiatric outcomes) are reported from primary studies cited in the review; however, the paper does not state that meta-analytic pooling or formal risk-of-bias assessment was performed.

Chemical and pharmacokinetic findings: Psilocybin (O‑phosphoryl‑4‑hydroxy‑N,N‑dimethyltryptamine) is a prodrug rapidly dephosphorylated to the active psilocin by phosphatases in gut and tissues. Psilocybin content in mushrooms ranges about 0.2–1% of dry weight. After oral administration in humans, psilocybin/psilocin appear in plasma within 20–40 minutes, with peak psilocin at 80–105 minutes; oral effects onset 20–40 minutes, peak at 60–90 minutes and typically last 4–6 hours. Psilocin plasma half‑life is reported as ~2.5 hours after oral dosing and ~1.23 hours after intravenous administration. Metabolism includes glucuronidation (UGT1A10 in intestine, UGT1A9 in liver) and oxidative pathways; about 80% of psilocin is excreted as glucuronide and most elimination occurs within 3 hours, complete by 24 hours. Receptor pharmacology and cellular effects: Psilocin acts primarily as an agonist at serotonin 5‑HT2A/C and 5‑HT1A receptors, with much higher affinity at human 5‑HT2A receptors than in rats. The 5‑HT2A agonism is considered necessary for hallucinogenic effects; other receptor interactions (including weak binding at dopamine and other receptors) have also been reported. In neuronal systems expressing 5‑HT2A, psilocybin increases immediate early gene expression (eg c‑fos and egr family), a pattern the authors discuss in the context of biased/heteromeric receptor signalling (5‑HT2A/mGlu2 complex) as a possible mechanism distinguishing hallucinogenic from non‑hallucinogenic 5‑HT2A agonists. Animal and behavioural pharmacology: Doses used in animal studies vary widely (0.25–10 mg/kg typical, up to 80 mg/kg in some experiments). In rodents and other species psilocybin/psilocin produce dose‑dependent locomotor inhibition, signs of serotonergic behavioural syndrome (eg head‑twitch), altered sensorimotor gating (prepulse inhibition, with dose and species dependence), impaired performance on certain conditioned tasks, and reduced aggression/dominance behaviours. In primates, high doses produce stereotyped scanning and behaviours suggestive of altered perception, consistent with visual hallucination‑like phenomena. Human psychotropic and neuropsychological effects: Psychedelic effects typically emerge above ~15 mg oral psilocybin (corresponding to plasma psilocin 4–6 ng/ml); the drug is ~45× less potent than LSD and ~66× more potent than mescaline. Low doses accentuate pre‑existing mood, medium doses induce controllable altered states, and higher doses produce strong psychedelic experiences including altered perception, synaesthesia, derealisation/depersonalisation, time distortions, thought disturbances and marked emotional variability. Psychometric instruments (eg Altered States of Consciousness scales, Adjective Mood Rating Scale) document increased visual phenomena, altered affect and transient cognitive/perceptual impairments; some effects persist for hours but generally resolve within 24 hours. Longer‑term subjective changes: Several controlled and follow‑up studies report that a single psilocybin session can produce enduring positive changes in attitudes, mood and life priorities, including increased ratings of personal meaning and spirituality and, in some participants who had mystical‑type experiences, sustained increases in the personality trait openness lasting longer than one year. Physiological and endocrine effects: Sympathetic stimulation is typically mild—mydriasis, modest rises in systolic/diastolic blood pressure (~10–30 mmHg) and heart rate increases (average 82–87 bpm, peaks reported up to 140 bpm). Psilocybin has minimal acute effects on ECG or body temperature. It transiently elevates prolactin and, at high doses, corticotropin, cortisol and thyrotropin, with hormones returning to baseline within about five hours. Toxicity, safety and dependence potential: Acute somatic toxicity is low. Reported LD50 values are high in rodents (psilocybin LD50 ~280–285 mg/kg; psilocin LD50 lower at ~75 mg/kg for rodents). The LD50/ED50 ratio cited is 641. Clinical trial doses have not exceeded 0.429 mg/kg. Fatalities following mushroom ingestion have typically resulted from hazardous behaviour rather than direct pharmacological toxicity. Psychologically, adverse outcomes include transient anxiety, panic, prolonged unpleasant experiences, rare psychotic reactions and very rare hallucinogen persisting perception disorder (HPPD). Prevalence estimates from pooled serotonergic hallucinogen exposures suggest prolonged psychiatric symptoms occur in ~0.08–0.09% of healthy subjects and ~0.18% of psychiatric patients; suicide attempts in psychiatric patients were reported at 0.12% with fatalities in ~0.04% in the sources summarised. Tolerance develops rapidly with repeated dosing, and evidence for physical dependence or craving is lacking; primate self‑administration data do not indicate rewarding effects. Direct mutagenicity/genotoxicity was not observed in one cited study, but teratogenicity data are lacking. Electrophysiology and neuroimaging: EEG/MEG studies report reduced absolute power and coherence (especially fronto‑temporal), decreased alpha power and desynchronisation; MEG shows broadband reductions in spontaneous cortical oscillatory power with large decreases in default mode network (DMN) regions. FDG‑PET studies at peak effect (15–25 mg p.o.) reported increased metabolism in lateral and medial prefrontal cortex (including anterior cingulate), temporomedial cortex and basal ganglia, and reduced uptake in thalamus; these metabolic increases correlated with positive psychotic‑like symptoms. In contrast, intravenous psilocybin fMRI studies documented decreases in BOLD and perfusion in subcortical and high‑level association regions and reduced connectivity among key hubs (eg posterior cingulate–mPFC), with subjective intensity predicted by decreases in anterior cingulate and mPFC activity. The authors discuss several possible reasons for PET/fMRI discrepancies, including timing relative to peak effects and vasoactive actions of psilocybin. Clinical applications: Historical clinical use in the 1950s–1960s included psycholytic/psychedelic psychotherapy across a range of conditions. Recent pilot and controlled studies summarised include low‑dose psilocybin (0.2 mg/kg) showing anxiolytic and antidepressant effects in terminal cancer patients, reports of transient benefit in obsessive‑compulsive disorder, and preliminary support for facilitation of smoking cessation and alcohol dependence through enhancement of spirituality and motivation. Case reports also suggest benefit in cluster headache. The review notes these findings are preliminary and further research is needed.

Tylš and colleagues interpret the assembled evidence as indicating that psilocybin is a well characterised psychedelic with considerable value both as an experimental probe of brain function and as a candidate therapeutic agent. They emphasise that the drug's primary action at 5‑HT2A receptors, its downstream effects on cortical excitability and connectivity, and its relatively favourable acute safety profile make it useful for investigating perception, emotion and the neurobiology of psychosis. The authors acknowledge important caveats and unresolved issues. They note discordant neuroimaging findings (PET versus fMRI), which may reflect differences in timing, vascular effects and the relationships between neuronal excitability, oscillatory desynchronisation and metabolic measures. The limits of serotonergic models of psychosis are discussed: while psilocybin reproduces many positive symptoms and some processing abnormalities, it lacks the negative symptom and cognitive deficit profile typical of chronic schizophrenia, and clinical antipsychotic efficacy crucially relies on D2 antagonism. Practical constraints are highlighted—rapid tolerance to psilocybin precludes chronic modelling in humans, and ethical and legal restrictions have historically limited clinical research. Safety uncertainties are also identified. Although controlled studies report low rates of serious adverse events, the reviewers point to sparse data on teratogenicity and a small but non‑zero incidence of prolonged psychiatric sequelae and HPPD. The Schedule I legal status is criticised as inconsistent with the accumulated evidence of potential medical utility and acceptable safety under supervision. Finally, the authors call for further controlled clinical trials to define therapeutic indications and mechanistic studies to reconcile imaging and electrophysiological findings.

The authors conclude that psilocybin has substantial research and therapeutic potential. They highlight the drug's well understood pharmacokinetics and pharmacodynamics, its favourable safety profile in controlled settings and an absence of evidence for addiction liability. Psilocybin is presented as a valuable tool for probing serotonergic contributions to perception and emotion, a useful experimental model with translational validity, and a promising candidate for clinical development in several psychiatric and neurological conditions, pending further rigorous research.