Effects of an international ceremonial ayahuasca retreat on the diversity and community composition of the human gut microbiome: A preliminary observational single-arm pre-post study
Lyanna R. Kessler, Lamya'a Dawud, John D. Sterrett, Nathan D. Andersen, Ahmed I. Elsayed, Jesse J. Gould, Kathryn M. Pate, W. Keith Campbell, Jim Grigsby, Brandon Weiss, Christopher A. Lowry
Brain Behavior and Immunity Integrative July 1, 2026 DOI: 10.1016/j.bbii.2026.100182 (opens in new tab)
Study at a glance
AI-extracted from the abstract| Characteristics | Observational single-arm pre-post study Randomized Placebo-controlled Double-blind Peer reviewed |
|---|---|
| Sample size | 22 |
| Population | U.S. Veterans and non-Veterans with varied PTSD symptoms |
| Intervention | Ayahuasca ceremonies |
| Duration | One-week retreat |
| Topics | Anxiety Ayahuasca |
| Keywords | Microbiome Observational study Mental health Diversity politics Neuroticism Mood Personality Clinical psychology Ecology Alpha diversity Mental illness Affect linguistics Dysbiosis Beta diversity |
| Key findings | Participation in an ayahuasca retreat was associated with reduced PTSD symptoms, shifts in gut microbiome community composition, and increased relative abundances of several short-chain fatty acid-producing bacteria, though alpha diversity did not change. |
Abstract
ABSTRACT Posttraumatic stress disorder (PTSD) is a mental health condition that can develop following exposure to a traumatic event, often with persistent anxiety and depressive symptoms. Recent research suggests the microbiome-gut-brain axis may play an important role in the etiology and pathophysiology of PTSD. At the same time, retreats incorporating psychedelic-assisted therapies have gained increasing attention as potential treatments for PTSD. However, little is known about how participation in these retreats may affect the gut microbiome. Therefore, we conducted a preliminary observational single-arm pre-post study investigating the diversity and community composition of the gut microbiome pre/post participation in a retreat involving ceremonial ayahuasca use, as well as associations between microbiome features, PTSD symptoms, and personality traits. Participants included U.S. Veterans and non-Veterans with varied PTSD symptoms ( N = 22). The international retreat took place over one week and included three ayahuasca ceremonies. PTSD Checklist for the DSM-5 (PCL-5) scores, Five-Factor Model (FFM) personality measures, and fecal samples were collected from participants pre/post retreat. Fecal samples were analyzed using 16S rRNA gene amplicon sequencing to assess gut microbiome diversity and community composition. PCL-5 scores decreased following the retreat, relative to baseline. Openness, Conscientiousness, Extraversion, and Agreeableness increased, while Neuroticism decreased. Measures of gut microbiome alpha diversity did not change. However, beta diversity analyses revealed a within-subjects shift in microbial community composition at the end of the retreat relative to baseline. Post retreat there was an increase in the relative abundances of several short-chain fatty acid-producing bacteria associated with gut health, including Lachnospiraceae UCG-004, Colidextribacter, Oscillospira, and Blautia. Although the study design has limitations and does not establish causality, it highlights within-subjects effects of participation in an ayahuasca retreat on the gut microbiome, effects that should be evaluated further in future large randomized double-blind, placebo-controlled clinical trials.