Selection bias can be strong. In a survey comparing psychedelic enthusiasts to a more general sample, enthusiasts reported markedly larger quality-of-life gains, with a between-group difference around d = 0.84.
Importantly, this gap remained even after adjusting for mindset, setting, motivation, and personality, and group membership was the strongest predictor of reported impact. These findings are summarized in a recent peer-reviewed report available on PubMed.
Caveats matter: the data come from a cross-sectional online self-report, so it does not show objective health changes or causality. The key takeaway is about study design and interpretation, not clinical effects.
Practically, studies that recruit many enthusiasts are likely to overstate benefits. Representative sampling and careful control of expectations and context are needed for a fairer picture. Bottom line: selection bias can materially inflate reported benefits.