Mind over matter: the microbial mindscapes of psychedelics and the gut-brain axis

The paper opens by situating the gut microbiota as a central component of the gut-brain axis, emphasising that the microbial metagenome considerably expands the host's biochemical repertoire and influences digestion, immune priming, enterohepatic circulation and neural signalling. Earlier research has documented consistent differences in faecal microbiota composition between healthy people and those with psychiatric or neurologic conditions, and causal links have been shown in animal models where behavioural traits can be transferred via faecal microbiota transplantation. At the same time, a resurgence in clinical and mechanistic psychedelic research has demonstrated substantial therapeutic potential of serotonergic psychedelics for disorders such as treatment-resistant depression, PTSD, OCD and addiction, with actions largely attributed to 5-HT2A receptor activation, altered large-scale brain connectivity and enhanced neuroplasticity. Caspani and colleagues note a gap in the literature: despite long-standing knowledge that many psychedelic compounds are plant-derived and that plant compounds can have antimicrobial properties, there is little direct research on how psychedelics interact with the gut microbiome and whether the microbiome contributes to psychedelics' psychological effects. The review therefore aims to synthesise existing evidence—both direct and indirect—on interactions between serotonergic psychedelics and the microbiota-gut-brain axis, to generate mechanistic hypotheses, identify knowledge gaps, and outline implications for personalised medicine and future research frameworks.

This manuscript is a narrative, hypothesis-generating review rather than a systematic review. The study does not report a formal, reproducible search strategy or pre-specified inclusion/exclusion criteria; instead, the investigators assembled direct and indirect evidence from human, animal and in vitro studies that bear on potential psychedelic–microbiome interactions. The authors explicitly acknowledge the speculative nature of parts of their synthesis and rely on related literature about serotonergic drugs (for example, SSRIs), microbial modulation of host serotonin, microbial effects on drug metabolism, and mechanistic studies of immune, neuroendocrine and neural effects. Evidence sources reported in the extracted text include preclinical animal experiments, in vitro immunological assays, human observational and experimental studies of psychedelics, studies of antidepressant–microbiome interactions, and microbiome intervention studies (for example probiotics) that relate to brain connectivity or neuroplasticity. The review integrates these strands into conceptual figures and a summary table of shared biological targets. Where direct data linking psychedelics to microbiome changes are scarce, the authors draw on mechanistic pathways (serotonergic signalling, host–microbial co-metabolism, immune and HPA axis interactions, vagal signalling) to build testable hypotheses about mediation and moderation roles for the microbiome.

The review collates a set of empirical and mechanistic findings that together suggest bidirectional interactions between serotonergic psychedelics and the gut microbiome. Direct studies are currently sparse: the authors identify a single, non-peer-reviewed rodent study reporting that oral psilocybin and an analogue increased Verrucomicrobia and Actinobacteria and decreased Proteobacteria in rat faecal samples, without changing overall alpha diversity. Complementary evidence derives from related domains: selective serotonin reuptake inhibitors (SSRIs) alter gut microbial composition and possess antimicrobial activity, and phytochemicals in psychedelic plants (for example compounds in Banisteriopsis caapi and Peyote) show antibacterial and antifungal properties in vitro. Several mechanisms by which the microbiome could shape psychedelic responses are described. Gut bacteria can directly metabolise xenobiotics and participate in host–microbial co-metabolism (for example enterohepatic recycling), thereby changing drug bioavailability, activity or toxicity. The bacterial enzyme tryptophanase can divert host tryptophan to indole, reducing substrate for serotonin synthesis, and microbial-derived inflammatory signals (for example LPS) can upregulate host indoleamine-2,3-dioxygenase (IDO), shifting tryptophan metabolism toward kynurenine. An animal model of maternal immune activation showed an enhanced hallucinogenic-like response to DOI, and taxa-level correlations were reported between certain bacterial families and cortical 5-HT2A receptor density (Ruminococcaceae and Candidatus saccharibacteria positively correlated; Lactobacillaceae negatively correlated), suggesting a potential microbial influence on receptor expression. The authors review shared biological targets through which psychedelic–microbe interplay might influence brain and behaviour. Immune modulation: psilocybin reduced TNF-α and IL-1β in an in vitro macrophage model; psilocin and DMT altered microglial markers in mice; DOI suppressed TNF-α in a 5-HT2A-dependent manner; ayahuasca reduced certain T lymphocyte subsets and systemic CRP in humans. HPA axis effects: several psychedelics (LSD, psilocybin, ayahuasca, DMT) increased ACTH and cortisol in humans, and DOI/DOB raised corticosterone in rats, linking psychedelic exposure to stress-axis activation which is itself responsive to microbiome state. Brain connectivity: psychedelics consistently decrease within-network connectivity of the default mode network (DMN) and increase cross-network connectivity; alterations in DMN connectivity have been associated with inflammatory markers and with specific bacterial taxa in clinical samples. Neuroplasticity: psychedelics modulate genes linked to plasticity, long-term potentiation (LTP) and synaptogenesis; psilocybin showed dose-dependent effects on hippocampal neurogenesis (increasing at 0.1 mg/kg, decreasing at 1.0 mg/kg in rodents), DMT and 5-MeO-DMT stimulated neurogenesis in vitro and in vivo, while LSD and DOI had no significant effects in some rat studies. Parallel microbiome literature shows germ-free or antibiotic-treated animals have impaired neurogenesis and that probiotics or faecal transfers can restore plasticity measures. The authors formalise three causal scenarios: (1) psychedelics act on the microbiome and microbial shifts mediate therapeutic effects; (2) the microbiome modulates psychedelic responses (for example via drug metabolism), acting as a moderator; (3) psychedelics alter psychological state and these neural/behavioural changes secondarily reshape the microbiome. They emphasise that all three scenarios could operate concurrently. In applied terms, the review highlights translational opportunities: known pharmacogenetic predictors (CYP2D6 polymorphisms affecting psilocybin and DMT metabolism; HTR2A variants influencing receptor function) illustrate how biological heterogeneity affects psychedelic response, and the authors propose that baseline microbial signatures might similarly predict efficacy or adverse effects and be targeted via probiotics, dietary interventions or other microbiome-directed strategies to optimise outcomes.

Caspani and colleagues interpret the assembled evidence as suggestive but preliminary: there is a plausible, multi-level rationale for interactions between serotonergic psychedelics and the gut microbiome, yet direct empirical data remain limited. They argue that both psychedelic compounds and the microbiome converge on shared physiological systems—serotonergic signalling, immune function, the HPA axis, large-scale brain networks and neuroplasticity—which could mediate or modulate therapeutic effects. Rather than asserting a single causal pathway, the authors endorse a systems-biology perspective that recognises the microbiome as a ‘‘convergence hub’’ integrating multiple biofeedback loops. Key limitations are acknowledged. The paper is not a systematic review and therefore does not claim exhaustiveness; evidence specific to psychedelics' effects on gut microbiota composition is scarce and includes non-peer-reviewed data. Many inferences are drawn from related pharmacology (for example SSRIs) or from preclinical models, and human observational findings can be confounded by lifestyle factors such as diet and exercise. The authors call for carefully designed future studies to address four priority areas: (1) identify gut microbes and microbial enzymes involved in psychedelic metabolism, (2) measure the effects of psychedelics on microbiome composition and function in humans and animals, (3) map baseline microbial profiles to heterogeneity in clinical and subjective outcomes, and (4) evaluate the safety and efficacy of microbiome-targeted interventions to optimise psychedelic therapies. Implications for clinical practice are framed cautiously. The investigators suggest that integrating microbiome assessment into psychedelic therapy research could enable stratification of patients, more precise dosing or route selection, and adjunctive microbiome-targeted strategies to enhance response and reduce adverse effects. They underscore the need for multi-modal, longitudinal and mechanistic studies to translate these ideas into evidence-based personalised approaches, while controlling for confounders and thoroughly assessing safety.

The review concludes that serotonergic psychedelics and the gut microbiota are likely to be linked through bidirectional, multi-level interactions, and that the microbiome may act both as a mediator and a moderator of psychedelic effects on brain function and mental health. Caspani and colleagues advocate for future investigations to adopt systems-biology, multimodal approaches that incorporate microbiome data when modelling psychedelic action. Such integrative research could inform new, personalised psychedelic treatment strategies tailored to an individual's multi-system biological signature.