Fecal transplant (FMT) for irritable bowel syndrome (IBS)

Development of the therapy

Interest in FMT for IBS grew from the observation that many patients develop IBS after a gut infection, and that the IBS microbiome is measurably different from healthy controls — lower diversity, altered ratios of Firmicutes to Bacteroidetes, and changes in methane-producing organisms.¹ Case series in the early 2010s reported symptom improvement, prompting several randomised controlled trials in Norway, Sweden, and elsewhere.

Therapeutic target

IBS is a disorder of gut–brain interaction with no single cause. The rationale for FMT is that the microbiome contributes to visceral hypersensitivity, altered motility, and low-grade immune activation — particularly in post-infectious IBS and in IBS with diarrhoea (IBS-D). Restoring a diverse donor community is hypothesised to normalise fermentation patterns, bile-acid metabolism, and gut-barrier function.

What the trials show

• Conflicting results: a Norwegian trial of colon-delivered FMT from a well-characterised "super-donor" reported response rates around 65–90% versus roughly 25% on placebo, while a Swedish study using frozen capsules found placebo outperformed FMT.²³
• Donor and route matter: outcomes appear highly sensitive to donor selection, stool preparation, and delivery method — more so than in C. difficile.
• Subgroup signals: post-infectious IBS and IBS-D look more promising than IBS with constipation, but no subgroup finding is yet definitive.
• Benefit, when seen, often fades over 6–12 months; maintenance dosing strategies are untested.

Possible side effects and risks

• Transient bloating, cramping, and altered stool pattern in the days after treatment
• Risk of worsening symptoms — some trial participants deteriorated relative to placebo
• Theoretical weight or metabolic effects, given the microbiome's role in metabolism
• The general long-term uncertainty that applies to any microbiome-modifying therapy

FAQ: FMT in IBS

Anyone with IBS considering FMT should first work through established therapies with a gastroenterologist and, if interested in FMT, look for a registered clinical trial.

References

1. Jeffery IB, et al. An irritable bowel syndrome subtype defined by species-specific alterations in faecal microbiota. Gut. 2012.
2. Johnsen PH, et al. Faecal microbiota transplantation versus placebo for moderate-to-severe irritable bowel syndrome: a double-blind, randomised, placebo-controlled, parallel-group, single-centre trial. Lancet Gastroenterol Hepatol. 2018.
3. Lahtinen P, et al. Effects of faecal microbiota transplantation in patients with irritable bowel syndrome. Aliment Pharmacol Ther. 2020.