The gut microbiome and MS: what research shows

What the gut microbiome is

In plain words: Microbes in the gut help shape the immune system, so researchers ask whether they play a part in MS.

Background from a scientific review

The gut microbiome is the large community of bacteria and other micro-organisms that live in the intestine. A 2022 scientific review describes how these microbes influence the immune system, including T cells and B cells, the immune cells involved in MS.^[1]

The same review concludes that carefully designed studies in large groups of people are still needed to understand which gut changes matter in MS.^[1]

What studies in people with MS have found

In plain words: On average, people with MS have somewhat different gut bacteria, but these studies cannot show which comes first.

Association studies in people

The largest study, iMSMS, compared 576 people with MS with 576 healthy people who lived in the same household, to reduce differences in diet and surroundings. Some bacteria were more common and others less common in MS. The gut bacteria also differed with the MS treatment people were taking.^[2]

Earlier studies of 60 people with MS and 43 healthy people, and of 71 untreated people with MS and 71 healthy people, also found differences in specific bacteria, without large changes in the gut community as a whole.^{[3], [4]}

A study of 34 pairs of identical twins, where only one twin had MS, found no major difference in the overall gut bacteria; some specific bacteria were more common in untreated twins with MS.^[5]

A 2025 study followed people with MS over two years and linked some gut bacteria and substances they produce with worsening disability. It shows associations, not causes.^[6]

A difference found in a study can be a cause of MS, a consequence of it, or the result of treatment, diet or other factors. The authors of one of these studies state that further research is needed to tell these apart.^[3]

What experiments in mice have shown

In plain words: In mice bred to develop an MS-like disease, gut bacteria can change that disease; this has not been shown in people.

Laboratory studies in mice

In a mouse strain that develops an MS-like disease on its own, the disease needed the normal gut bacteria to start.^[7]

When gut bacteria from twins with MS were transferred into such mice, the mice developed the MS-like disease more often than mice that received bacteria from their healthy twins.^[5]

In another experiment, germ-free mice that received gut bacteria from people with MS and were then given an MS-like disease by immunisation had more severe symptoms than mice that received bacteria from healthy people.^[4]

A 2025 study of a larger group of identical twins, one with MS and one without, found more than 50 groups of bacteria that differed. In mice, bacteria taken from the small intestine of twins with MS triggered the MS-like disease more often; two bacteria from the same family were the likely active ones.^[8]

These are experiments in mice. They guide research, but no treatment study in people has confirmed that changing gut bacteria treats MS.^[1]

Treatments being tested: probiotics, faecal transplants and diet

In plain words: Probiotics, faecal transplants and diet changes have been tested only in small, short studies; none has been shown to reduce relapses or disability.

Small, short studies in people

Probiotics: in a study of 9 people with relapsing MS and 13 healthy people, everyone took a probiotic for two months, without a placebo group. Gut bacteria and some immune markers in the blood changed; the study was not designed to show an effect on relapses or disability.^[9]

In a 2024 trial without a placebo group, people with MS took a probiotic or a prebiotic (fibre that feeds gut bacteria) for six weeks. Both were well tolerated. The probiotic improved self-reported bowel control, but no other symptom measure improved over that time.^[10]

Faecal transplants (faecal microbiota transplantation) move stool from a healthy donor into another person's gut. In a Canadian pilot study, 9 people with relapsing MS received monthly transplants. The study stopped early, so it could not answer its main question, about inflammatory markers in the blood. One person developed hives linked to the transplant; the authors reported no severe or life-threatening side effects in this small group.^[11]

A ClinicalTrials.gov search on 1 November 2026 found no large controlled faecal transplant trial in MS. The four registered studies specific to MS listed between 1 and 20 participants each, and three of them were terminated.^[12]

Diet changes gut bacteria. In mice, intermittent fasting eased the MS-like disease, partly through gut bacteria. In people with relapsing MS, the 18-month NAMS trial found that fasting and ketogenic diets did not lead to fewer new lesions on MRI than a standard healthy diet.^{[13], [14]}

Other probiotic trials, including a retracted paper, are covered in the supplements and diets guide. →

Risks

In plain words: Faecal transplants outside a regulated study can transmit serious infections; probiotics carry more risk when the immune system is weak.

Safety information from regulators

In 2019 the US Food and Drug Administration (FDA) reported that two adults with weakened immune systems developed invasive infections with drug-resistant E. coli after an investigational faecal transplant; the donor stool had not been tested for these bacteria. One of them died. The FDA then required donor stool in such studies to be tested for drug-resistant bacteria.^[15]

Probiotics are usually well tolerated by healthy people, but the risk of serious infection is greater in people who are severely ill or whose immune system is weakened.^[16]

What it means for you today

In plain words: No treatment aimed at gut bacteria has been shown to slow MS; this research is still at an early stage.

Summary of the evidence

Studies in people show differences in gut bacteria, and studies in mice show that gut bacteria can affect an MS-like disease. Treatment studies in people have so far been small and short, and did not measure, or did not show, fewer relapses or less disability.^{[2], [9], [10], [11]}

The FDA warning above concerned donor stool that had not been tested for drug-resistant bacteria. Discuss probiotics, faecal transplants and major diet changes with your neurologist, especially if your treatment affects your immune system. Never stop or change prescribed treatment on your own.^[15]