Background and aims: Interest in psychedelic research has grown significantly in recent years and the naturally derived substance psilocybin, in combination with therapy, has shown promising results as a treatment for a range of psychiatric conditions. However, the negative effects and risks of psilocybin-assisted treatment are not well-established. The purpose of this study was to investigate the potential negative effects of psilocybin-assisted psychological interventions in both the short and long term.

Method: Semi-structured interviews were conducted with eight psychedelic treatment providers and facilitators. Their content was thematically interpreted.

Results: Three themes of short-term negative effects were identified. They included negative reactions to psilocybin dosing sessions, undesirable processes in the therapeutic relationship, and difficult self-experiences. Four themes of long-term negative effects were identified. They included destabilization of the client, difficulties adapting to life post-treatment, complications in the treatment relationship, and undesirable outcomes.

Conclusions: These results highlight the multifaceted challenges clients may face, emphasizing the need for thorough pre-intervention assessment and post-intervention support. The findings both confirm previous research and highlight new aspects that can contribute to increased safety and be relevant for clinical implementation. Further rigorous research is needed to ensure safety, establish ethical guidelines, and optimize the positive effects of these experimental medicines. Integrating various research approaches and types of measurements will be vital to further our understanding of negative effects of psychedelic-assisted therapy.