A placebo is a dummy treatment. ClinicalTrials.gov defines it as something that looks the same, and is given the same way, as the treatment being studied, but contains no medicine (ClinicalTrials.gov). Comparing a drug with a placebo shows how much of a change comes from the drug itself rather than from time, attention or hope.
Placebo groups often improve. In a microdosing study that 191 people completed, both the microdose and placebo groups got better, with no significant difference between them (Szigeti et al., 2021). A 2025 meta-analysis found that control groups in psilocybin depression trials improved less than control groups in antidepressant trials, which may mean psilocybin’s effect is overestimated (Hieronymus et al., 2025).
Psychedelics make a plain placebo hard to use, because most people can tell they got nothing. That is why researchers try active placebos or very low doses, and why functional unblinding is a standing concern. Health Canada warns that people who join a trial to get a new treatment may not receive it (Health Canada).
Sources
- ClinicalTrials.gov: Learn About Studies
- Szigeti et al., eLife, 2021
- Hieronymus et al., JAMA Network Open, 2025
- Health Canada: Clinical trials and drug safety
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