How Might Psilocybin Support Change? A Multi-Level Look at the Science

People who come to us asking about psilocybin usually have a version of the same question: how would this actually help? Interest has grown quickly in whether classic psychedelics can loosen the rigid patterns behind depression, anxiety, trauma responses, addiction and other long-standing struggles. What gets lost in the headlines is that there is no single answer to how they work.

The research points to effects at several levels at once: changes in individual neurons, in the circuits that filter information, in how large brain networks communicate, and in how a person holds their own beliefs and habits of mind. These accounts overlap more than they compete. None of them is settled, and none of them describes something the medicine does on its own. The people who seem to do best are the ones who were well prepared, well supported during the session, and did real work afterward.

Before getting to mechanisms, it helps to be clear about what is being explained.

First: what human trials have shown

Most of the clinical evidence for psilocybin is in depression, and the trials are still small by the standards of psychiatric medicine.

In a small randomized trial (27 adults with major depression, 24 of whom completed it), two psilocybin sessions with psychological support produced large reductions in symptoms compared with a delayed-treatment group (Davis et al., 2021).
In the largest trial so far, 233 people with treatment-resistant depression received a single dose of 25 mg, 10 mg or 1 mg. The 25 mg group improved more than the 1 mg group at three weeks. Whether that difference lasted was less clear, and adverse events, including suicidal thoughts and behavior, occurred across all groups (Goodwin et al., 2022).
In a trial comparing psilocybin with the antidepressant escitalopram over six weeks, the two did not differ significantly on the main depression measure. Some secondary measures favored psilocybin, but those were not the trial’s main test (Carhart-Harris et al., 2021).

Research in anxiety around serious illness, alcohol use, smoking and other conditions is earlier still. The fair reading: psilocybin with support shows real promise in depression, durability is still being studied, and much of what follows is about explaining effects that are themselves still being measured.

There is also a non-pharmacological explanation that any honest account has to include. In most psilocybin trials, participants can tell whether they received the drug, which makes true blinding nearly impossible. People who enroll often expect to benefit. Researchers have argued that expectancy and unblinding may account for part of the effects seen in trials, and that current study designs can’t fully separate them from the drug’s action (Muthukumaraswamy et al., 2021). That doesn’t mean the effects aren’t real. It means the size of the drug’s own contribution is still an open question.

With that framing, here are the main accounts of how psilocybin might support change, from the smallest scale to the largest.

Level 1: Neurons and plasticity

The best-supported biological account is about neuroplasticity: the brain’s ability to grow and reshape connections between neurons. The practical upshot, explained below, is that psilocybin appears to make the brain temporarily more able to rewire, and that this rewiring and the psychedelic experience itself may not be the same process.

The receptor behind the experience

Psilocybin is converted in the body to psilocin, which acts mainly on a serotonin receptor called 5-HT2A. These receptors are especially dense on the long branching dendrites of large output neurons in the cortex (layer 5 pyramidal neurons), and densest overall in the association cortex that handles complex thought and self-reflection.

The clearest human evidence that this receptor drives the experience is decades old: when volunteers were given ketanserin, a drug that blocks 5-HT2A, before psilocybin, most of psilocybin’s effects on perception and consciousness were blocked (Vollenweider et al., 1998).

Growth in neurons

Laboratory work over the past decade has found that psychedelics can drive structural change in cortical neurons:

In cultured neurons and in rodents, psychedelics promoted the growth of dendrites, dendritic spines (the small protrusions where synapses form) and new synapses. This work linked the effect to signaling pathways long associated with plasticity, including BDNF, its receptor TrkB, and mTOR (Ly et al., 2018).
In mice, a single dose of psilocybin increased the number and size of dendritic spines in the frontal cortex by roughly 10% within a day, and some of those new spines were still present more than a month later (Shao et al., 2021).
The plasticity effect in rodent cortical neurons appears to depend on 5-HT2A receptors located inside the cell, not only on its surface (Vargas et al., 2023). Psychedelics cross cell membranes easily; serotonin does not. When researchers got serotonin inside the cell, it produced plasticity too. The difference seems to be access, which may help explain why a psychedelic can produce effects that the brain’s own serotonin ordinarily doesn’t.

This receptor story also bears on a common question. Long-term use of SSRI antidepressants is thought to reduce 5-HT2A signaling, which is one proposed reason some people report a blunted psilocybin experience while taking them. What that means for any individual is a question for their prescriber; our post on psilocybin and SSRIs covers the evidence on both sides.

An open question: which receptor does the rewiring?

The picture isn’t tidy. One research group reported that LSD and psilocin bind directly to TrkB, the BDNF receptor, and that in mice the plasticity-related effects held up even when 5-HT2A signaling was blocked, while the hallucination-like behavior did not (Moliner et al., 2023). If that holds, the rewiring and the psychedelic experience could run through different receptors. The finding is contested, awaits independent replication, and sits uneasily beside the intracellular 5-HT2A account above. Both come from animal studies.

Can you get the plasticity without the experience?

Researchers have produced compounds that promote plasticity in the lab without the behavioral signs of a psychedelic experience. Tabernanthalog, a modified relative of ibogaine, showed plasticity and antidepressant-like effects in rodents (Cameron et al., 2021). Whether this carries over to humans is unknown. The field is openly divided on whether the subjective experience is necessary for lasting benefit: some researchers argue that it is (Yaden & Griffiths, 2021), others that it may not be (Olson, 2021). For now this is a live debate, not a finding.

A “critical period” that reopens

Early in development, the brain passes through critical periods, stretches of time when certain kinds of learning happen unusually easily. In mice, psilocybin, LSD, ketamine, MDMA and ibogaine all reopened a critical period for social reward learning, and how long it stayed open tracked how long each drug’s acute effects last in people (Nardou et al., 2023). It is a striking result and a mouse result. It is one reason clinicians think the days and weeks after a session matter so much.

A note on other compounds: ketamine and MDMA work through different primary targets (the glutamate NMDA receptor and serotonin release, respectively), yet show plasticity-related effects too, including in the critical-period study above. They are outside Colorado’s natural medicine program and outside what we offer. We mention them because the convergence on plasticity, across drugs that work very differently, is part of why researchers take this account seriously.

Bottom line: plasticity is currently one of the stronger biological explanations for why effects might outlast the session. In people, the role of 5-HT2A in the acute experience is well established; the evidence for rewiring comes mostly from cells and animals, and the exact pathways are still debated.

Level 2: Circuits that filter information

A second set of accounts looks at specific brain circuits, mainly to explain what happens during the experience.

The thalamic filter (CSTC model). The thalamus acts as a relay and gatekeeper, regulating how much sensory and internal information reaches the cortex. The cortico-striato-thalamo-cortical (CSTC) model, developed largely by Franz Vollenweider and colleagues, proposes that psychedelics loosen this gating. More information reaches the cortex with less top-down filtering, which may help explain the intensified perception and unusual associations people describe (Vollenweider & Preller, 2020). In human imaging, LSD changed the thalamus’s connections with the cortex, and a 5-HT2A blocker prevented those changes, which is partial support for the model (Preller et al., 2018).

The claustrum (CCC model). The claustrum is a thin sheet of neurons beneath the cortex with dense 5-HT2A receptors and connections to nearly every cortical region. It appears to help coordinate cortical networks, particularly when a task requires focus. The cortico-claustro-cortical (CCC) model proposes that psychedelics disrupt this coordinating role, contributing to the loosening of the brain’s usual network structure (Doss et al., 2022). In people, psilocybin reduced claustrum activity and altered its connections with major brain networks (Barrett et al., 2020).

These circuit models are good at explaining acute perceptual and cognitive shifts. On their own, they say less about why change might last.

Level 3: Large-scale brain networks

Brain imaging lets researchers look at how whole networks behave. Several observations have recurred:

More varied brain activity. The entropic brain hypothesis proposed that psychedelics increase the entropy of brain signals (Carhart-Harris et al., 2014). Here “entropy” is a statistical measure of variety, not disorder: brain activity becomes less predictable and covers a wider range of patterns than in ordinary waking consciousness.
Less separation between networks. Normally, brain networks are fairly distinct from one another. Under psychedelics they become less segregated and more cross-connected.
Changes in the default mode network. The default mode network (DMN) is active during self-referential thinking: rumination, mind-wandering, the running narrative about who we are. In an intensive study of seven healthy adults, psilocybin strongly desynchronized brain networks during the session, most of all the DMN, and one connectivity change, between the DMN and the hippocampus, was still measurable weeks later (Siegel et al., 2024).

The finding most directly tied to clinical outcome comes from two depression studies: a small open-label study and the trial comparing psilocybin with escitalopram. After psilocybin, brain networks became less modular (more globally integrated), and greater integration was associated with greater improvement. Escitalopram did not produce the same change (Daws et al., 2022).

Bottom line: a review of psychedelic brain-imaging studies found that results have been inconsistent across studies, samples are often small, and analysis methods vary widely (McCulloch et al., 2022). Network findings are among the most cited in the field and still need replication. They describe conditions that may make later reorganization possible; they do not prove it.

Level 4: Beliefs, habits of mind and psychological rigidity

The most clinically useful models work at the level of experience. They are also the most speculative.

REBUS: relaxed beliefs

REBUS (“relaxed beliefs under psychedelics”), proposed by Robin Carhart-Harris and Karl Friston, starts from the idea that the brain constantly predicts its experience using deeply held assumptions, called priors (Carhart-Harris & Friston, 2019). Some priors are about the physical world. Others are beliefs like “I am a burden” or “people leave.”

REBUS proposes that psychedelics temporarily reduce the confidence the brain places in its highest-level beliefs. With those beliefs held more loosely, incoming information from emotion, memory and the senses carries more weight than usual. That creates an opening to revise beliefs that ordinarily feel like facts.

Loosening cuts both ways. A belief held more loosely can be revised in a healthier direction, or replaced by a new belief that is just as rigid. Which happens depends heavily on what comes next.

Canalization

A related idea borrows an image from developmental biology: a ball rolling down a landscape of valleys. Over time, and often in response to adversity, patterns of thought and behavior carve deeper valleys that become harder to leave. This account proposes that much psychopathology involves this kind of canalization: patterns that may once have been protective but have become rigid (Carhart-Harris et al., 2023).

In this account, psychedelics temporarily flatten the landscape. Ruts become shallower, and other paths become reachable. Whether a new path is actually taken, and kept, depends on what happens afterward.

Pivotal mental states

Pivotal mental states are brief, highly plastic states that can occur under extreme stress or in moments of insight, in which rapid psychological change becomes possible (Brouwer & Carhart-Harris, 2021). The proposal is that psychedelics engage the same systems that evolved for these states. This also helps explain why the same state that can open something valuable can be destabilizing without the right support.

What the experience itself predicts

Some features of the acute experience track with outcomes. The best studied is mystical-type experience: a sense of unity, transcendence of time and space, and deep meaning. In treatment-resistant depression, people who reported stronger mystical-type experiences tended to improve more (Roseman et al., 2018). Emotional breakthrough and a sense of psychological insight have also tracked outcomes in research.

These are associations, not proof of cause. Each could reflect something else, such as how ready a person was for change. They are strong candidates for part of how change happens, and they remain candidates.

Bottom line: these models are influential and useful for thinking about what a session can open up. The direct evidence behind them is still preliminary and mixed. They are best read as frameworks, not established mechanisms.

The part the medicine can’t do alone

Every level above describes a temporary opening: neurons more able to rewire, circuits less filtered, networks less rigid, beliefs held more loosely. None of them says what fills that opening. That depends on everything around the medicine.

Screening comes first. The same openness that makes change possible is why some people should not take psilocybin, or should do so only with additional precautions, and why a person’s circumstances on the day matter. Our page on safety and screening explains the process, and our guide to who shouldn’t take psilocybin lays out the common rule-outs.

Preparation shapes what someone brings into a session: their intentions, their understanding of what may happen, their trust in the people supporting them. Our page on how to prepare for a psilocybin journey covers what that looks like in practice.

The setting and the relationship shape the session itself. Context is arguably not an add-on to psychedelic effects but part of the mechanism, and the more open the brain becomes, the more context matters (Carhart-Harris et al., 2018). Psychedelics also increase suggestibility: in one study, LSD made healthy volunteers more responsive to suggestion (Carhart-Harris et al., 2015). The words, expectations and presence of the people in the room carry more weight than usual, which is a responsibility as much as a tool. In one psilocybin trial for depression, a stronger therapeutic relationship was associated with more emotional breakthrough during the session and better outcomes afterward (Murphy et al., 2022). The physical environment and music matter as well; see what happens during a session and the role of music.

Integration is where insights become change. If the plasticity and canalization accounts are right, the days and weeks after a session are when new patterns take hold or the old ones reassert themselves. Integration also does a quieter job: sorting which realizations hold up. Insights under psychedelics can feel certain without being accurate, and researchers have argued that psychedelics can produce compelling false insights alongside true ones (McGovern et al., 2024). Talking an insight through, testing it against your life and relationships, and acting on the ones that survive is part of how change becomes durable. Our psychedelic integration page describes how that work is structured at FRTC.

Putting it together

No single model explains how psilocybin might support change yet. A fair summary of the evidence today:

Strongest biological support: 5-HT2A activity drives the acute experience in people, and psychedelics promote plasticity in cells and animals, with the exact rewiring pathways still debated.
Promising but in need of replication: less segregated, more flexible brain network activity, including one clinical link between network integration and improvement in depression.
Useful frameworks with preliminary evidence: REBUS, canalization and pivotal mental states, plus experience-level predictors like mystical-type experience.
Still unresolved: how much of the benefit in trials comes from the drug itself versus expectancy, and how long benefits last.

What these accounts share is the idea that the medicine makes change more possible, not inevitable. Individual responses vary widely, the research is moving fast, and nothing here is medical advice. The responsible path is legal, supervised use with careful screening, skilled support and real integration.

Learn more

If you’re considering psilocybin in Colorado, start with our natural medicine services, which explains how sessions, preparation and integration work at FRTC. For the legal framework, see Colorado’s natural medicine program. If you’re unsure whether it’s a good fit right now, is psilocybin therapy right for you? covers readiness and timing.

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