One of the easiest ways to make this subject vague is to talk about psychedelics as if that word alone tells us enough. It does not. Different substances get grouped together under one label, but
they do not work the same way,
do not feel the same way,
and do not fit the same therapeutic model.
That matters clinically. If someone says “psychedelics help depression” or “psychedelics are being used in trauma treatment,” the next question should be: which psychedelic? Psilocybin, LSD, DMT, MDMA, and ketamine are not interchangeable. Some are classical serotonergic psychedelics – some are not. Some are being studied mainly for depression, others for PTSD, others for different indications entirely. Treating them as one thing makes the field harder to understand, not easier.
Classical psychedelics
The clearest shared category is what are usually called classical psychedelics. These include substances such as psilocybin, LSD, DMT, ayahuasca, and 5-MeO-DMT. What broadly links them is their action on the serotonin 2A receptor (5-HT2A), even though each substance still has its own pharmacological profile and subjective character. To break it down, this means they act on one of the brain’s serotonin systems involved in perception, meaning-making, and how rigidly we hold on to ordinary patterns of thought. That interaction alludes to how these substances can alter how a person sees, feels, and interprets both inner and outer experience.
This is also where my own research sits most directly. My work in this field has focused particularly on psilocybin, and that remains the area I know in greatest depth. Once ingested, psilocybin is converted into psilocin, the active compound that most directly produces the experience. It acts on serotonin systems in the brain, especially those involved in perception, emotion, and the sense-making processes through which we organize experience.
LSD overlaps with psilocybin in some important ways, but it yields a different experience. It also interacts with dopamine receptors, which is one reason it should not simply be treated as “basically the same thing”. DMT, likewise, belongs in the classical family, but has its own profile; taken orally on its own it is rapidly broken down, whereas in ayahuasca it is combined with MAO-inhibiting plants, which is what allows the effects to occur by ingestion. 5-MeO-DMT is again related, but not the same: its receptor affinities differ from DMT, and that seems to matter for intensity and phenomenology.
What classical psychedelics broadly seem to share is not just receptor action, but a cascade of effects that follows: changes at molecular and synaptic levels, shifts in functional connectivity across brain networks, and psychological changes that can be intense, meaningful, and at times therapeutically relevant. The effects are generally linked to neural plasticity, increased global functional connectivity, and altered top-down processing. In other words, these substances seem able to temporarily loosen rigid mental patterns, making perception less ordinary, beliefs more flexible, and (therefore) new perspectives more possible.
Non-classical psychedelics
Then there are substances often discussed in the same breath, but which, technically, do not belong in the same category.
Ketamine is the obvious example. It is often included in psychedelic conversations because it can produce profound changes in consciousness and has real therapeutic relevance, especially in depression. But pharmacologically it is not a classical psychedelic. Ketamine works through a different brain system, one more closely involved in learning, flexibility, and the brain’s ability to update entrenched patterns. That helps explain both its dissociative feel and its unusually fast antidepressant effects in some people. It is more accurate to describe it as a dissociative anesthetic that can produce psychedelic-like effects at specific doses.
That difference matters. Ketamine is not simply “another psychedelic for depression.” It represents a different route into altered experience and a different therapeutic model. It also already occupies a different clinical space: esketamine received regulatory approval in Portugal for treatment-resistant depression in 2025, which makes it very different from substances still largely confined to research settings.
MDMA also belongs in this broader field, but again not as a classical psychedelic. It is more accurately described as an empathogen or entactogen. Its core mechanism involves increasing serotonin release and affecting dopamine and norepinephrine systems as well. In plain language, it increases brain chemicals involved in mood, energy, bonding, and social safety. That helps explain why MDMA is often linked less with perceptual distortion and more with reduced fear, greater trust, and a stronger sense of emotional openness. It has also been linked to oxytocin and vasopressin release, which helps explain why its clinical interest has been especially strong in PTSD, where fear, broken trust, impaired sense of social safety, and memory reconsolidation come up so often.
Why these differences matter
These distinctions are not academic nitpicking. They shape four things that readers should actually care about.
1. Mechanism
If one substance primarily acts through 5-HT2A (e.g. psilocybin) and another through NMDA blockade (e.g. ketamine), you are not dealing with the same intervention in any meaningful pharmacological sense. More simply: two substances may both alter consciousness, but they are not doing it in the same way, and that affects what kind of experience and treatment model they may support.
2. Subjective experience
Different psychedelics may all alter consciousness, but never quite in the same manner. Classical psychedelics are more strongly associated with perceptual shifts, altered meaning, ego changes, and changes in sensory and cognitive processing. MDMA is more closely linked with empathy, bonding, and fear reduction. Ketamine may involve dissociation, altered self-representation, and rapid antidepressant effects through a different path. In practical terms, the experience of “a psychedelic” can vary enormously depending on which substance is being discussed, not to mention the external circumstances like setting, intention setting, etc.
3. Therapeutic use
Psilocybin has been especially prominent in depression research and remains the most studied psychedelic compound in clinical settings. MDMA has become central to PTSD research. Ketamine has an already established clinical role in depression. If we flatten all of that into “psychedelics for mental health,” we lose the actual map of what kind of benefits certain substance can promise.
4. Safety and expectations
Different substances yield different trips: durations, intensities, contraindications, and psychological demands. Even within the classical family, effects are influenced profoundly by variables such as mindset, environment, tolerance, route of administration, and individual differences. That is one reason responsible work in this area cannot rely on general enthusiasm alone. In real life, this means that the same label tells you very little about what a person may actually go through, what support they may need, or what kind of risk makes sense to consider.
Where I’m going next
This is the first distinction I want readers to keep in mind: not all psychedelics are the same kind of substance, and not all of them belong in the same conversation in the same way.
That does not weaken the field. It strengthens it. It lets us think more clearly, read the evidence more honestly, and resist the kind of flattening that turns a serious area of inquiry into trend language.
In the next post, I want to move from the substances themselves to the mind more directly: what these compounds can do to perception, emotion, connection, meaning, and self-experience.
References:
Andersen, K. a. A., Carhart‐Harris, R., Nutt, D. J., & Erritzoe, D. (2020). Therapeutic effects of classic serotonergic psychedelics: A systematic review of modern‐era clinical studies. Acta Psychiatrica Scandinavica, 143(2), 101–118. https://doi.org/10.1111/acps.13249
Askariyan, K., Joghataei, M. T., Dehghan, S., Nohesara, S., Pour, L. R., Mohammadi, M. H., & Ahmadirad, N. (2025). An overview of psilocybin, LSD, MDMA, and ketamine in revitalizing psychedelic-assisted therapy: Insights, limitations and future directions. Progress in Neuro-Psychopharmacology and Biological Psychiatry, 141, 111461. https://doi.org/10.1016/j.pnpbp.2025.111461
Cummins, B. R., Billac, G. B., Nichols, D. E., & Nichols, C. D. (2025). 5-HT2A receptors: Pharmacology and functional selectivity. Pharmacological Reviews, 77(4), 100059. https://doi.org/10.1016/j.pharmr.2025.100059
De Vos, C. M. H., Mason, N. L., & Kuypers, K. P. C. (2021). Psychedelics and Neuroplasticity: A Systematic review unraveling the biological underpinnings of psychedelics. Frontiers in Psychiatry, 12, 724606. https://doi.org/10.3389/fpsyt.2021.724606
Jelen, L. A., & Stone, J. M. (2021). Ketamine for depression. International Review of Psychiatry, 33(3), 207–228. https://doi.org/10.1080/09540261.2020.1854194
Siegel, J. S., Subramanian, S., Perry, D., Kay, B. P., Gordon, E. M., Laumann, T. O., Reneau, T. R., Metcalf, N. V., Chacko, R. V., Gratton, C., Horan, C., Krimmel, S. R., Shimony, J. S., Schweiger, J. A., Wong, D. F., Bender, D. A., Scheidter, K. M., Whiting, F. I., Padawer-Curry, J. A., . . . Dosenbach, N. U. F. (2024). Psilocybin desynchronizes the human brain. Nature, 632(8023), 131–138. https://doi.org/10.1038/s41586-024-07624-5
