Ayahuasca Ceremonies Shifted Gut Bacteria and Cut PTSD Scores by 28% in New Study

If you suspect your mental health and your gut are tangled together in ways medicine hasn't fully mapped yet, you're tracking one of the most active frontiers in psychiatric research. A new preliminary study published in Brain, Behavior and Immunity Integrative adds a provocative early data point: a week-long ceremonial ayahuasca retreat appeared to measurably shift participants' gut microbiome composition, and those same participants reported significantly lower PTSD symptom scores by the end of the week. The findings are investigational, the sample was small, and there were no control groups, so treat everything here as hypothesis-generating, not settled science.

Key takeaways:

• A seven-day ayahuasca retreat reduced average PTSD symptom scores from 41.9 to 30.2, a 27.9% drop, among the 12 participants with paired pre/post psychological assessments (of 22 enrolled).
• Gut microbial community composition shifted significantly within subjects, with increases in several bacteria that produce short-chain fatty acids (SCFAs), compounds linked to reduced inflammation.
• Alpha diversity (the raw richness and variety of species in the gut) did not change.
• This was a single-arm observational study with no placebo or control group; the retreat environment, diet, and travel all could have contributed to the results.

Ceremonial Ayahuasca Reduced PTSD Symptom Scores by Nearly 28%

The study by Kessler, Lamya'a, Sterrett, Andersen and colleagues enrolled 22 U.S. veterans and non-veterans with varying levels of PTSD symptoms, recruiting through the Heroic Hearts Project. Participants traveled to rural Costa Rica for a seven-day retreat that included three ceremonial ayahuasca sessions. Ayahuasca is a plant-based brew, traditional to Amazonian indigenous groups, made from Banisteriopsis caapi (which contains beta-carboline compounds that act as monoamine oxidase inhibitors, or MAOIs, meaning they block an enzyme that would otherwise break down the brew's active molecules) and Psychotria viridis (which contains DMT, or dimethyltryptamine, a potent serotonin-receptor agonist that produces the brew's hallucinogenic effects).

PTSD symptom severity was measured using the PCL-5 (PTSD Checklist for DSM-5), a validated 0-to-80 scale where a score of 31 to 33 is the threshold for probable PTSD. Of the 22 enrolled, 12 participants had complete paired pre/post psychological data; among them, the average score dropped from 41.9 before the retreat to 30.2 afterward, crossing below the probable-PTSD threshold as a group average. Personality measures also shifted: openness, conscientiousness, extraversion, and agreeableness all increased, while neuroticism decreased significantly.

Gut Microbiome Composition Shifted, With Gains in SCFA-Producing Bacteria

The gut microbiome findings are where this study breaks new ground. The gut-brain axis, the bidirectional communication network linking gut microbial communities to brain function and mood regulation, is increasingly recognized as relevant to psychiatric conditions including PTSD. The researchers collected fecal samples on day one and day five of the retreat and analyzed them using 16S rRNA gene amplicon sequencing (a technique that identifies bacteria by reading a specific segment of their genetic code, like a barcode). Of the 22 enrolled, 13 participants had usable fecal samples for the microbiome analysis.

Alpha diversity, which measures how many distinct species are present and how evenly they are distributed within a single person's gut, did not change significantly. But beta diversity, a measure of how much the overall microbial community composition shifted within each participant over time, did change significantly. In plain terms: the kinds of bacteria present, and their relative proportions, were measurably different by the end of the retreat even if the total variety wasn't.

Several bacteria that produce short-chain fatty acids (SCFAs, small molecules that gut bacteria make when they ferment dietary fiber, and that help regulate intestinal inflammation and gut-brain signaling) increased in relative abundance. These included Lachnospiraceae UCG-004, Colidextribacter, Oscillospira, and Blautia. The Eubacterium eligens group, a well-characterized SCFA producer, also increased significantly. One bacterium in the Romboutsia genus decreased. Notably, prior case-study evidence on psychedelic ceremonies has also observed reductions in pro-inflammatory gut bacteria and increases in butyrate-producing species alongside mental health improvements, suggesting a pattern worth investigating more rigorously.

How the Study Was Conducted and Why the Design Limits What We Can Conclude

This was a preliminary observational single-arm pre-post study, meaning everyone got the same intervention and there was no comparison group. Of the 22 people enrolled, only 12-13 contributed usable data to the two analyses, a small sample by any standard. The retreat protocol included not just three ayahuasca ceremonies but also dietary restrictions (a whole-foods diet, no alcohol, no recreational drugs, and restrictions on certain medications) and international travel from the United States to Costa Rica, both of which could independently shift gut microbiome composition.

The researchers did not confirm the purity or microbial profile of the ayahuasca brew itself, did not collect follow-up fecal samples after participants returned home, did not verify dietary adherence before the retreat, and did not measure participant expectations, a known variable in psychedelic research. The cohort was also largely Caucasian and high-income, which limits how broadly the findings generalize. Advances in gut microbiome research methodology, including larger multicenter designs and shotgun metagenomics, will be needed before these signals can be interpreted with confidence.

What This Could Mean If You Are Managing PTSD or a Gut-Related Condition

For readers tracking their own PTSD, anxiety, depression, or inflammatory gut conditions like IBS or IBD, the honest answer is: this study raises interesting questions but cannot tell you what to do. The SCFA-producing bacteria that increased are associated with lower intestinal inflammation and healthier gut-brain signaling, and SCFA levels have been linked to PTSD severity in prior research. That is a biologically plausible connection. But plausible is not the same as established.

It is worth knowing, as context, that widespread misconceptions about gut microbiota include the idea that any increase in a particular bacterial group is automatically beneficial. The clinical significance of the compositional shifts observed here is genuinely unknown. Nobody in this study was prescribed a microbiome intervention, and no finding here supports self-treating PTSD or any other condition with ayahuasca or supplements targeting these bacterial strains.

What is worth asking a doctor about: whether emerging research on the microbiome-gut-brain axis is relevant to your treatment plan, and whether psychedelic-assisted therapy trials are open to you if you have treatment-resistant PTSD.

Open Questions: Controls, Long-Term Follow-Up, and the Role of the Brew Itself

The authors are explicit that future work must include rigorous control arms: a group attending the same retreat without consuming ayahuasca, a group following only the dietary protocol, and a healthy-control comparison. Long-term fecal sampling after participants return to their home environments would reveal whether any microbial shifts persist or simply reflect the travel and dietary context of the retreat week.

The mechanism is also genuinely unclear. Beta-carboline compounds in B. caapi have shown antibacterial properties in laboratory settings. DMT's target receptor, 5-HT2A (a serotonin receptor), is expressed in the gut wall and influences gastrointestinal motility, which could indirectly affect the microbial environment. Whether the brew, the diet, the intensive ceremonial setting, the travel, or some combination is driving the microbiome signal remains an open question.

Common Questions

Did the study prove ayahuasca improves PTSD? No. The PCL-5 scores dropped significantly among the participants with complete data, but without a control group it is impossible to separate the effect of ayahuasca from the retreat environment, dietary changes, group support, or participant expectations. These findings are preliminary and observational.

Is it safe to try ayahuasca for PTSD based on this research? This single small study is not sufficient basis for a personal health decision. Ayahuasca is a potent psychoactive brew with real risks, including dangerous interactions with certain medications (particularly antidepressants that affect serotonin). Any consideration of psychedelic-assisted therapy should involve a qualified clinician.

What are short-chain fatty acids and why do they matter? SCFAs are small molecules produced when gut bacteria ferment dietary fiber. They help regulate intestinal inflammation, support the gut lining, and communicate with the brain via the gut-brain axis. Lower SCFA levels have been associated with higher PTSD symptom severity in prior research, though whether that relationship is causal remains unproven.

Will there be a follow-up study with controls? The authors explicitly call for future studies with placebo conditions, diet-only and travel-only comparison groups, and long-term follow-up sampling. No specific trial has been announced in this paper.

References

1. Lyanna R. Kessler, Lamya’a M. Dawud, John D. Sterrett, et al. (2026). 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. Brain Behavior and Immunity Integrative. https://doi.org/10.1016/j.bbii.2026.100182
2. Pegah Ataei, Hamidreza Kalantari, Tamara S. Bodnar, Raymond J. Turner (2026). The gut–brain connection: microbes’ influence on mental health and psychological disorders. Frontiers in Microbiomes. https://doi.org/10.3389/frmbi.2025.1701608
3. Payton Follestad, Patricia Filbin, Brice Thompson, et al. (2025). Gut–Mind Interactions in Psychedelic Healing: A Case Study Assessing the Effects of Huachuma and Ayahuasca on the Mind and Microbiome. Journal of Restorative Medicine. https://doi.org/10.14200/jrm.2025.0006
4. James L Alexander, Benjamin H Mullish, Linda Thomas, et al. (2026). Recent advances in our understanding of the gut microbiome: an analysis from the Gut Microbiota for Health Expert Panel of the British Society of Gastroenterology. Gut. https://doi.org/10.1136/gutjnl-2026-338252
5. Priyankar Dey (2025). All That Glitters Ain’t Gold: The Myths and Scientific Realities About the Gut Microbiota. Nutrients. https://doi.org/10.3390/nu17193121