Risk of bias is a rating that systematic reviews give each included study. Reviewers check how participants were randomized, whether the blind held, whether data went missing, and whether all planned outcomes were reported. A high risk of bias doesn’t mean a result is wrong, but it means it could be.
In psilocybin research, blinding is the recurring problem. A 2024 BMJ systematic review rated all included psilocybin depression trials at moderate risk of bias and called for research on expectancy (Metaxa and Clarke, 2024). A 2021 review argued that blinding and expectancy problems likely overestimate effect sizes across psychedelic trials (Muthukumaraswamy et al., 2021). A 2022 set of recommendations for more rigorous psychedelic trials includes active placebos and measuring how well blinding worked (Aday et al., 2022).
Readers can do a short version themselves: who knew what, who dropped out, what was the comparison, and was the reported result the primary endpoint?
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