What happens when an emerging programme of medical research overlaps with a surging social movement? In this article we draw on the anthropological term ‘chemosociality’ to describe forms of sociality born of shared chemical exposure. Psychedelic administration in the context of recent clinical trials appears to have been particularly chemosocial in nature. We argue that one consequence is that psychedelic-assisted therapy (PAT) clinical research trials tend to breach key assumptions underlying the logic of causal inference used to establish efficacy. We propose the concept of dark loops to describe forms of sociality variously emerging from, and impacting participant experiences in, PAT trials. These dark loops are not recorded, let alone incorporated into the causal pathways in the interpretation of psychedelic trial data to date. We end with three positions which researchers might adopt in response to these issues: chemosocial minimisation where research is designed to attenuate or eliminate the effects of dark loops in trials; chemosocial description where dark loops (and their impacts) are openly and candidly documented and chemosocial valorisation where dark loops are hypothesised to contribute to trial outcomes and actively drawn upon for positive effect. Our goal is to fold in an appreciation of how the increasingly-discussed hype surrounding psychedelic research and therapeutics continues to shape the phenomena under study in complex ways, even as trials become larger and more rigorous in their design.