The Science
Neuroplasticity, the most over-used word in the field
Everyone says psychedelics rewire the brain. Here is what that means at the level of a single synapse, what has only been shown in mice, and why the mechanism still doesn't agree with itself.
"Psychedelics rewire the brain" is now said so often that it has stopped meaning anything. It is also, in a narrow and well-evidenced sense, true. Worth separating the two.
What plasticity means when you zoom in
Neurons connect through synapses, and those connections are not fixed. Dendritic spines, the small protrusions where connections form, can grow, strengthen, shrink and disappear. Growth factors such as BDNF and signalling pathways such as mTOR govern much of this. Neuroplasticity, stripped of the mysticism, means this ongoing structural remodelling: the brain physically changing how its cells are wired together.
It is not exotic. It is happening while you read this. What is interesting about psychedelics is the claim that they accelerate it.
The evidence, and the species it comes from
The foundational paper is Ly and colleagues, "Psychedelics Promote Structural and Functional Neural Plasticity", published in Cell Reports in June 2018 from David Olson's lab. It also coined the useful word psychoplastogen, from roots meaning mind, moulded and producing: a compound that rapidly promotes structural plasticity.
Then in 2021, Shao and colleagues, working in Alex Kwan's lab, imaged the frontal cortex of living mice before and after a single dose of psilocybin, published in Neuron. They saw increased dendritic spine density and larger spine heads, and the changes persisted for around a month.
Two things about all of that. First, it is mice. The structural imaging that makes this field's headlines cannot currently be done in a living human brain. Second, and more interesting, the same lab has since complicated its own story. In a 2025 Nature paper, Shao and colleagues showed psilocybin grew spines in two different cell types in mouse frontal cortex, but silencing one type eliminated the behavioural benefit while silencing the other made no difference. Spine growth was present without the benefit. That is the cleanest demonstration yet that plasticity and therapeutic effect are not the same thing.
The argument the field is actually having
It is a proper, named, unresolved disagreement, and it was staged as a pair of papers in the same journal in 2021.
David Olson argued that the benefit comes from the plasticity, that the hallucinogenic and plasticity-promoting properties can be pulled apart chemically, and that the subjective experience may be beside the point. His lab and others have since produced several non-hallucinogenic compounds that appear to promote plasticity in rodents.
David Yaden and Roland Griffiths argued the opposite: that the neurobiology is necessary but not sufficient, and that the subjective effects do real causal work. Note that they do not deny plasticity matters. Framing this as "plasticity versus the trip" is a false binary; the live question is whether the experience adds something the biology alone does not.
Meanwhile the mechanism refuses to converge. Whether the 5-HT2A receptor is required for the plasticity effect has been answered yes by some studies and no by others. As one 2023 review in Neuropsychopharmacology put it, whether neuroplasticity mediated the clinical changes "remains an open question."
How to read the next claim you see
Three questions will get you most of the way. Was it a mouse or a person? Was it a measure of the brain changing, or of the person getting better, because those are different findings? And was it published, or announced? At least one widely circulated human result for a non-hallucinogenic psychoplastogen comes from a company press release covering 18 participants with no stated placebo control, which is not evidence that the approach works in people.
None of this scepticism undercuts the core idea, which remains one of the most quietly hopeful facts in neuroscience: the adult brain keeps rebuilding itself, and nothing about how you currently think is structurally permanent. That was true before psychedelics were interesting, and it will stay true whichever way the mechanism debate lands.
Neuroplasticity in Progress puts the same idea in three small panels. Down in the dark, pushing up, out into the light.
