An agonist is a molecule that binds a receptor and activates it, the way the body’s own messenger would. A 2016 review reports a consensus that psychedelics are agonists or partial agonists at brain 5-HT2A receptors (Nichols, 2016). A 2017 review describes both psilocybin and psilocin in those terms (Dinis-Oliveira, 2017).
Its opposite is an antagonist, which occupies the receptor without switching it on. Researchers use that contrast as a test: if blocking a receptor stops an effect, the receptor is probably needed. The 5-HT2A antagonist ketanserin blocked psilocybin’s effects in a 1998 study (Vollenweider et al., 1998).
Agonists are not limited to serotonin. Muscimol, from the fly agaric, is described as a potent agonist at GABA receptors (Günther et al., 2026), and a 2009 paper proposed DMT as an agonist at the sigma-1 receptor (Fontanilla et al., 2009). Binding to a receptor in a lab dish is not the same as producing an effect in a person, which is why reviews separate binding data from human findings.
Sources
- Nichols, 2016, Pharmacological Reviews: Psychedelics
- Dinis-Oliveira, 2017, Drug Metabolism Reviews
- Vollenweider et al., 1998, NeuroReport
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