The pharmacology of psilocybin

Psilocybin-containing mushrooms are described as one of the major hallucinogenic drugs of abuse worldwide, with a growing user population and a range of reported psychiatric complications. Although synthetic psilocybin (Indocybin®) was manufactured and used experimentally in the 1960s, the authors note that until recently pharmacological data were limited and widely dispersed across older and newer studies. Passie and colleagues set out to collate and review the available pharmacological information on psilocybin, emphasising human pharmacokinetic and pharmacodynamic findings from recent psychophysiological studies alongside earlier experimental and clinical reports. The review explicitly excludes a detailed characterisation of the complex psychopathological phenomena induced by the drug and focuses on pharmacology and safety-related data.

The paper is a narrative review that brings together historical and contemporary pharmacological data on psilocybin rather than a systematic review with a described search strategy. The authors draw on a mixture of experimental animal studies, early non-blind human investigations, and more recent double-blind, placebo-controlled human trials and neuroimaging studies. Specific analytic techniques cited in the extracted text include HPLC metabolite assays and PET imaging with radioligands. Detailed inclusion or exclusion criteria, database searches, search dates, and formal risk-of-bias assessment are not reported in the extracted text. Consequently, the synthesis appears to be qualitative and descriptive, integrating pharmacokinetic, pharmacodynamic, somatic toxicity, and receptor-binding information from diverse study designs and species.

Chemical identity and history: Psilocybin (4‑phosphoryloxy‑N,N‑dimethyltryptamine) is a substituted indolealkylamine in the tryptamine class. It was isolated from Psilocybe mexicana by Albert Hofmann in 1957 and synthesised in 1958. The compound is found in many mushroom species globally. Psychic effects: At medium oral doses (reported range 12–20 mg), psilocybin produces a controllable altered state characterised by affective stimulation, heightened introspection, perceptual changes (illusions, synaesthesia), altered thought and time sense, and features akin to hypnagogic experiences and dreaming. Onset and duration reported in human studies vary: effects typically last 3–6 hours after oral dosing, with full effects arising within about 70–90 minutes. The review reports threshold and dose ranges: a subjectively detectable sympathomimetic effect may occur at 3–5 mg orally, and full psychotropic effects commonly with 8–25 mg, though interindividual variability is emphasised. Somatic and physiological effects: Animal data include a reported mouse intravenous LD50 of 280 mg/kg. Across multiple species, doses around 10 mg/kg subcutaneously produced autonomic signs such as mydriasis, piloerection, heart and respiratory irregularities, and modest hyperglycaemia and hypertension; motor behaviour was described as muted. Early human EEG studies (small samples) showed reductions in alpha and theta frequencies and changes in visual evoked potentials. Human physiological studies cited (including double-blind trials) document only discrete changes in blood pressure and pulse, and some transient reductions in leukocyte counts between the second and fourth hour after administration. Endocrine measures (cortisol, prolactin, growth hormone) were reported as not significantly affected in a small double-blind study. Mutagenicity testing in mice (micronucleus test) showed no evidence of chromosomal damage, though the authors note that a single test cannot definitively exclude mutagenic potential. Pharmacokinetics: Oral absorption is substantial for labelled psilocybin, with detectable plasma levels within 20–40 minutes on an empty stomach in human studies. The review reports that psilocin (the dephosphorylated derivative) appears in plasma after about 30 minutes and that psychological effects arise with some reported plasma threshold (the extracted text gives an unclear plasma concentration value). A significant first‑pass hepatic effect is described, with most psilocybin converted to psilocin prior to reaching systemic circulation; rodent and human data presented support psilocybin behaving as a prodrug for psilocin. After a rapid rise in psilocin plasma levels, a plateau of roughly 50 minutes was reported followed by a slower decline ending by about 360 minutes in oral studies. The mean elimination half-life of psilocin is reported as approximately 50 minutes, but the review notes considerable interindividual variability and a shorter apparent half-life and subjective effect duration with intravenous administration in one trial (numerical values are reported in the extracted text). Renal excretion includes glucuronidated metabolites and a small proportion (reported as 3–10%) of unchanged psilocybin; about two-thirds of renal excretion of psilocin occurs by three hours in the studies cited. Tolerance and dependence: Early comparative studies cited report that 100 µg (the extracted text indicates 100 m g but appears to refer to microgram/unclear units) of psilocybin was considered equivalent to 1 µg LSD and 1000 µg mescaline in one set of experiments; significant tolerance with repeated use is documented, as is cross‑tolerance with LSD. The authors state that physical dependence does not appear to develop. Pharmacodynamics and neurobiology: Neuroimaging (PET) studies are summarised: one study found no increase in global brain metabolism, while another (Vollenweider et al.) reported regional metabolic increases in frontal cortex (about 24%), anterior cingulate (25%), temporal‑medial cortex (25%) and basal ganglia (19%), with smaller increases in sensorimotor and occipital cortices and a relative decrease in thalamic metabolism; an enhanced frontal–occipital metabolic gradient and pronounced right frontotemporal activation were highlighted. Receptor binding data indicate high affinity at 5‑HT2A receptors (Ki reported as 6 nM) and lower affinity at 5‑HT1A receptors (Ki reported as 190 nM). Unlike LSD, psilocybin and psilocin reportedly show no affinity for dopamine D2 receptors. Pharmacological blockade studies cited include ketanserin (a preferential 5‑HT2A antagonist) which fully blocked psychotomimetic effects in a double‑blind study (n = 15 with ketanserin pre‑treatment), supporting a primary role for 5‑HT2A activation. Pretreatment with the D2 antagonist haloperidol also reduced psychotomimetic effects in other work, suggesting a possible secondary involvement of dopaminergic neurotransmission. Mechanistic hypotheses discussed include inhibition of dorsal raphe serotonergic neurons leading to activation of locus coeruleus noradrenergic neurons (possibly explaining some perceptual alterations) and disrupted cortex–thalamus feedback loops that could ‘‘open’’ thalamic sensory gating and contribute to frontal hyperactivity. Safety and limitations of the evidence: Overall, the review characterises psilocybin as exhibiting low physiological toxicity and being relatively well tolerated in the studies surveyed. However, the authors emphasise that many primary studies are old, frequently small and non‑blinded, and that contemporary safety pharmacology studies are generally lacking. They caution that the main hazards arise from psychotomimetic effects, particularly in vulnerable individuals and in uncontrolled settings.

Passie and colleagues interpret the assembled literature as indicating that psilocybin is principally a prodrug for psilocin, which mediates the compound's psychoactive effects primarily via 5‑HT2A receptor activation. The authors place weight on more recent double‑blind and neuroimaging studies that link subjective effects to regional cortical metabolic changes, notably frontal hyperactivity and altered thalamocortical dynamics. The review situates these findings alongside receptor pharmacology and animal experiments that point to functional interactions among serotonergic, dopaminergic and noradrenergic systems; such interactions may help explain the phenomenology of perceptual changes and the ability of both 5‑HT2A antagonists and dopamine antagonists to attenuate psychotomimetic effects. Clinical safety is viewed cautiously: the physiological toxicity reported is low, but the evidentiary base is limited by old study designs, small sample sizes, variable analytical methods for detecting parent drug, and a paucity of rigorous contemporary safety pharmacology studies. Key limitations acknowledged in the text include the predominance of early non‑blinded studies, variability in plasma concentrations and pharmacokinetics across individuals, and insufficient analytical sensitivity historically to demonstrate the absence of parent psilocybin in plasma. The authors recommend that complications are most likely to arise from psychotomimetic effects in susceptible individuals and that better‑designed safety and pharmacological investigations are required to firm up conclusions about clinical risk and mechanism. They also note the relevance of emerging therapeutic research (for example in compulsive disorders) as a reason to clarify psilocybin's pharmacology and safety profile.