An adverse event is any unfavourable health change that happens to a participant during a study, recorded whether or not the treatment caused it. Comparing rates between groups helps show which events are linked to the drug. A serious adverse event is one that, for example, threatens life or leads to hospital admission.
Adverse events are common in psilocybin trials. In a 233-person trial, they occurred in 77% overall (Goodwin et al., 2022). In a 104-person trial, 44 of 50 people on psilocybin and 33 of 54 on niacin reported at least one through day 43 (Raison et al., 2023). In an MDMA trial for PTSD, severe adverse events were reported by 5 of 53 people on MDMA and 2 of 51 on placebo (Mitchell et al., 2023).
Trials screen out many people at higher risk, such as those with psychosis, so their safety figures may not apply to everyone. Rare harms show up only in larger numbers or in case reports. A microdosing review found that adverse events were mostly unreported or mild (Page et al., 2026).
Sources
- Goodwin et al., NEJM, 2022
- Raison et al., JAMA, 2023
- Mitchell et al., Nature Medicine, 2023
- Page et al., Frontiers in Psychiatry, 2026
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