Acute psychological and physiological effects of psilocybin in healthy humans: a double-blind, placebo-controlled dose-effect study

Psilocybin (PY, 4-phosphoryloxy-N,N-dimethyltryptamine) is the principal psychoactive constituent of certain hallucinogenic mushrooms and acts primarily at serotonergic receptors, notably 5-HT2A, with additional affinity at 5-HT1A and 5-HT2C subtypes. Previous human and animal work has implicated 5-HT2A activation in alterations of perception, mood and cognition, and suggested downstream interactions with other neurotransmitter systems such as dopamine. Although a small number of human studies have examined somatic effects after single doses, the dose–response relationships of psilocybin across psychological (including altered states of consciousness and attention) and physiological domains had not been characterised comprehensively in a controlled within-subject design. Hasler and colleagues therefore set out to map acute, dose-dependent psychological and physiological effects of psilocybin in healthy volunteers and to assess its short-term somatic safety. The study aimed to measure subjective experience (using a multi-dimensional altered-states scale), mood, sustained attention, cardiac electrophysiology, cardiovascular and thermoregulatory responses, endocrine markers and standard clinical chemistry across four ascending doses plus placebo, thereby informing both mechanistic inquiry and safety considerations for human challenge studies.

Design and participants: The investigators used a double-blind, within-subject, placebo-controlled design. Eight healthy volunteers (four men, four women; mean age 29.5 years, range 22–44) completed five experimental sessions, each at least 2 weeks apart, receiving placebo and four doses of psilocybin in random order. Screening excluded current major psychiatric disorder, relevant family history, and significant prior illicit drug use; personality outliers on selected subscales were also excluded. Female participants were studied in the early follicular phase and none were taking oral contraceptives. Intervention and dosing: Pharmaceutical-grade psilocybin capsules (1 mg and 5 mg) and matching lactose placebo were prepared under quality control. Doses were scaled to body weight and reported in the extracted text as: very low dose (VLD) = 45 g/kg, low dose (LD) = 115 g/kg, medium dose (MD) = 215 g/kg, and high dose (HD) = 315 g/kg. Both participants and investigators were blind to condition; dosing took place at approximately the same time of day to reduce circadian confounds. Psychological measures and timing: Subjective altered states were assessed with the 5D-ASC (five dimensions plus a global ASC score), completed at 150 and 300 minutes after administration and covering the intervals 0–150 min and 150–300 min respectively. Mood was measured with the Adjective Mood Rating Scale (AMRS) at 10, 95, 275 minutes and 24 hours. Sustained attention was evaluated using the Frankfurt Attention Inventory (FAIR) at 140 minutes post-dose. Physiological monitoring and assays: Continuous two-channel Holter electrocardiography (EKG) was recorded for 24 hours starting 1 hour before dosing. Blood pressure and axillary temperature were measured at baseline and at 5, 30, 60, 90, 120, 165 and 210 minutes; mean arterial pressure (MAP) and area-under-the-curve (AUC) values were calculated. Blood for clinical chemistry, haematology and hormones (TSH, prolactin (PRL), ACTH and cortisol (CORT)) was sampled 20 minutes before dosing and at 105 and 300 minutes. Assay methods and coefficients of variation for each analyte are reported in the extracted text. Statistical analysis: Given the within-subject design, repeated-measures univariate two-way ANOVAs with treatment (placebo and four doses) and observation time as factors were used to test effects; significant main effects or interactions were followed by Tukey HSD post-hoc pairwise comparisons. A significance threshold of P<0.05 was applied. The within-subject approach reduces intraindividual variability when comparing drug conditions.

Subjective time course and general effects: Volunteers typically reported first subjective changes 20–40 minutes after ingestion, with peak effects at 60–90 minutes and a gradual return to baseline by about 6 hours. Even the reported threshold dose (VLD, 45 g/kg) produced clear psychoactive effects for most subjects, characterised by mild drowsiness and intensified mood. LD, MD and HD doses produced dose-dependent changes in mood, sensory perception (from intensified perception and illusions to complex visual hallucinations and synesthesia) and alterations in time, space and self. Most MD and HD experiences were described positively; however, one male participant (age 23) experienced pronounced transient anxiety and fearful ego dissolution after HD psilocybin that resolved without pharmacological intervention by 6 hours. Altered states of consciousness (5D-ASC) and mood (AMRS): Scores on all 5D-ASC core dimensions increased dose-dependently. Only MD and HD psilocybin produced marked loosening of ego boundaries (oceanic boundlessness and anxious ego dissolution) and pronounced perceptual changes (visionary restructuralization). AMRS subscales showed increases in general inactivation, emotional excitability and dreaminess following MD and HD; dreaminess remained elevated at 24 hours after HD in one measure. Attention (FAIR): Performance on the FAIR attention test at 140 minutes was substantially impaired after MD and HD psilocybin, with ‘‘performance value’’ and ‘‘continuity value’’ reduced to roughly 50% of placebo, VLD and LD performance. Scores after VLD and LD were similar to each other, as were MD and HD, reflecting qualitative grouping of lower versus higher doses. Cardiac electrophysiology, blood pressure and temperature: Holter EKG analyses revealed no differences across conditions in the examined parameters (mean/min/max heart rate, extrasystoles, S–T changes, pauses). Axillary body temperature was not significantly affected by any dose (ANOVA F4,28=0.94, P=0.452). Mean arterial blood pressure showed a moderate elevation only at 60 minutes following HD administration; overall AUC analyses did not indicate persistent alterations. Neuroendocrine results: Repeated-measures ANOVA showed a significant main drug effect only for PRL (F4,28=6.41, P=0.0009), while dose×time interactions were significant for TSH (F8,56=3.95, P=0.0009), PRL (F8,56=4.68, P=0.0002), ACTH (F8,56=4.23, P=0.0005) and CORT (F8,56=2.43, P=0.025). Tukey HSD post-hoc tests indicated significant elevations at 105 minutes following HD psilocybin: TSH (P<0.01), PRL (P<0.001), ACTH (P<0.01) and CORT (P<0.05). PRL was already significantly increased after MD (P<0.01). By 300 minutes, all hormone concentrations had returned to pre-dose levels. The authors note that only the mean PRL value at 105 minutes after HD (28.0 ± 7.5) exceeded the laboratory’s stated normal range (3.4–24.1 µg/l as reported in the extracted text). Clinical chemistry and haematology: Apart from transient increases in two liver enzymes (ASAT and GGT) at 105 minutes following HD psilocybin, no significant effects on clinical-chemical markers were observed. The transient enzyme elevations remained within normal physiological ranges and returned to baseline by 300 minutes. Complete blood counts before the first and after the last experimental day did not differ and no haematological values were outside normal ranges.

Hasler and colleagues interpret their findings as evidence that psilocybin produces dose-dependent alterations of perception, affect and ego-related functions mediated principally via 5-HT2A receptor activation, with higher doses producing marked loosening of ego boundaries and pronounced visual phenomena. They note that lower doses produced transient intensifications or distortions of perception rather than sustained hallucinatory experiences, and that subjective effects typically resolved within 6–8 hours. In terms of cognitive effects, the investigators observed a substantial impairment of sustained attention at medium and high doses on the FAIR test, but acknowledge that decreased motivation under the drug could have contributed to poorer task performance. The authors propose a possible compensatory noradrenergic activation at the highest dose (315 g/kg as reported) that may explain a relative reduction in vigilance impairment at that dose compared with the medium dose. Physiologically, the study documents short-lived endocrine activation at peak drug effects, with dose- and time-dependent increases in TSH, PRL, ACTH and cortisol, and a statistically robust PRL elevation. The investigators situate these findings within known serotonergic regulation of the hypothalamo–pituitary–adrenal and hypothalamo–pituitary–thyroid axes and suggest that 5-HT2 receptor stimulation can activate these neuroendocrine pathways. They further emphasise that hormone levels returned to baseline by 300 minutes, and that no persistent somatic abnormalities were found. The authors acknowledge two key limitations: the small sample size (n=8), which reduces power to detect small effects especially on psychological measures, and the choice of the highest dose which they consider conservative for safety reasons and therefore may not capture effects apparent at larger doses. Finally, they conclude that single-dose psilocybin in this controlled setting produced no evidence of acute somatic hazard in healthy volunteers, but recommend that individuals with cardiovascular disease or untreated hypertension avoid psilocybin and that administration occur in controlled clinical settings because transient anxiety and ego-disruption can occur even in mentally stable participants.

Our investigations provided no cause for concern that administration of PY to healthy subjects is hazardous with respect to somatic health. However, as our data revealed tendencies of PY to temporarily increase blood pressure, we advise subjects suffering from cardiovascular conditions, especially untreated hypertension, to abstain from using PY or PY-containing mushrooms. Furthermore, our results indicate that PY-induced ASC are generally well tolerated and integrated by healthy subjects. However, a controlled clinical setting is needful, since also mentally stable personalities may, following ingestion of higher doses of PY, transiently experience anxiety as a consequence of loosening of ego-boundaries.