A small clinical trial suggests oral fecal microbiome transplantation (FMT), sometimes called “poop pills,” may raise peanut tolerance in some adults with peanut allergy, but the approach remains experimental.
The study, published August 5 in Science Translational Medicine, enrolled 15 adults and tested whether taking capsules of fecal material from healthy donors could reduce allergic reactions to peanut protein. Participants swallowed 36 frozen capsules over one or two days. The trial had two parts: 10 people received FMT without prior antibiotics, and 5 received a short antibiotic course before transplantation.
Overall, 6 of 15 participants showed an increased threshold for reacting to peanut after a single FMT dose: 3 of 10 in the no-antibiotic group and 3 of 5 in the antibiotic-pretreated group. The treatment was generally well tolerated in this small study; investigators reported no serious adverse events.
Immune and microbiome analyses linked the clinical improvements to changes in regulatory immune cells and bacterial metabolites. Participants who gained tolerance had increases in RORγt+ regulatory T cells, a population that helps control allergic immune responses. Researchers also observed higher levels of certain bile acid metabolites in both these human participants and in mice given their microbiomes.
To test causality, the team transferred microbiomes from human responders into mice. Mice that received these transplants were protected from peanut allergy and showed the same rise in RORγt+ regulatory T cells. Further experiments implicated a specific bacterial function: when researchers removed bile salt hydrolase activity from Bacteroides species in mice, the protective effect disappeared, suggesting this enzyme and bile-acid metabolism may contribute to reduced allergy in the animal model. That mechanistic finding has not yet been demonstrated in humans.
Investigators described the trial as a landmark early-phase effort showing that a microbiome-based therapy can alter food-allergy outcomes and illuminate how gut bacteria, metabolites, and the immune system interact to shape response.
Experts not involved in the study urged caution. The trial was small, open-label, and early-stage: six responders versus nine nonresponders means the signal is preliminary and not ready to change clinical practice. Larger, controlled trials are needed to confirm efficacy, determine who is likely to benefit, and develop a standardized, safer product than whole-stool capsules.
The researchers said future work will test a more refined product called microbiota transplantation therapy (MTT), which contains purified microbes with minimal nonmicrobial fecal material and can be stored at home. They plan longer dosing and follow-up to see if higher doses and sustained administration produce more durable protection. With sufficient funding and positive results in larger trials, a clinically useful microbiome therapy for food allergy could potentially emerge over the next decade or more.
Until then, clinicians emphasize that standard care remains the same: strict avoidance of known allergens, an emergency action plan, and specialist allergy management. If further studies identify specific bacteria or metabolites responsible for protection, those defined therapies would be the next step toward a safe, standardized treatment.