Background Psilocybin is a classic psychedelic that acutely alters brain functional connectivity. These changes are linked to therapeutic doses and subjective effects, with some evidence that changes persist beyond acute drug administration. However, the effects of lower doses on sustained connectivity changes remain unclear.

Methods Ten healthy volunteers received three psilocybin doses (between 5 and 20 mg) in a randomized and blinded order, with at least one week between doses. Resting-state functional magnetic resonance imaging was completed at baseline and one week after a single dose. Functional connectivity changes were analyzed in relation to dose and altered conscious states at both the level of individual brain region connections (edges) and resting-state networks.

Results Dose-dependent changes in 77 edges (76 increases, 1 decrease, of 1275 possible) were observed, but none survived multiple-comparison correction. At the network level, we observed one dose-dependent between-network increase (of 21 possible), and one dose-dependent within-network increase (of seven possible); the latter surviving correction. Alterations in conscious state were positively associated with widespread connectivity changes (dose-adjusted), with many network-level associations surviving correction. These directional patterns showed that lower doses and smaller conscious state alterations were linked to decreased connectivity, whereas higher doses and greater conscious state alterations were linked to increased connectivity.

Conclusions Dose level and acute subjective effects were positively associated with multiple functional connectivity changes one week after a low-to-moderate psilocybin dose. Further research is warranted to characterize these sustained effects and their therapeutic relevance to inform studies adopting similar dosing regimens in clinical cohorts.

R.K. is on the advisory board of Psychae Institute, a non-profit psychedelic research institute. R.K. has received grant funding from the Wellcome Trust, the Medical Research Council (U.K.), the National Health and Medical Research Council (Australia), and the Weary Dunlop Foundation for research on FND. R.K. receives royalties from Guildford Press for a book chapter on FND. O.C. has received funding from The Perception Restoration Foundation. C.B. has received funding from the Graham Burrows Travelling Scholarship, University of Melbourne, and honoraria for FND speaking engagements by Epworth Healthcare (Australia). A.B. and O.D. have no relevant disclosures to declare.

Australian New Zealand Clinical Trials Registry, ACTRN12621000560897

https://doi.org/10.1176/appi.prcp.20240047

This work was supported by the Medical Research Future Fund (R.K., O.C., and A.B., grant number MRF2012410) and the RANZCP Foundation, the Royal Australian and New Zealand College of Psychiatrists (C.B.).

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The Austin Health Human Research Ethics Committee of Austin Health, Australia, gave ethical approval for this work (HREC/57390/Austin-2020).

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The authors wish to thank the Melbourne Brain Centre, Austin Health (Australia), for their support and assistance with MRI data acquisition.